Live Displays

The LiveDisplays plug-in allows real-time visualisation histograms, based on pointers to TH1D and TH2D objects (see the ROOT web pages for documentation on these). The live displays are created and initialised at run-time, can be updated in real time as the run progresses, and can be saved to a variety of graphics and data formats both during and after the run. Auxiliary methods allow to include comparisons to reference data from experimental measurements and/or saved runs, using file read-in methods which are adaptible to almost any conceivable (simple) column-based data format, in particular those used by the HepDATA repository. If the binning of the reference data differs from that of the base TH1D object, a flag can be set to show a rebinned version of the latter instead, recasting it into the binning of the reference measurement. It is also possible to include and display a variety of additional content including uncertainty variations, basic chi-squared calculations, ratios, and to re-initialise/recover from previous runs. Note: for help on ROOT, please consult the ROOT documentation and forums, not the PYTHIA authors. Few of us are ROOT users and none of us are ROOT developers, therefore this plug-in is provided "as is" and any non-PYTHIA-specific questions or problems you may encounter when using or linking this plug-in to ROOT should be addressed to the ROOT development team.


If you use plots created by the LiveDisplays plugin in a publication, we ask that you include a reference to [GKS11] in which the functionality underlying the this plug-in was first developed. It would also be appropriate to include a reference to the ROOT software package, which the plug-in is based on.


The PYTHIA collaboration is providing this plug-in as a simple utility for real-time histogram monitoring, which can also be used as a convenient way to produce near-publication-quality plots. Developing this facility is, however, not the core focus of our collaboration, and is provided "as is". Please do not send us any unsolicited "feature requests". For extensions or bug fixes that are of general interest, feel free to develop further capabilities and send us (documented and complete) examples of working and validated code, following the CODINGSTYLE guidelines that are distributed with PYTHIA 8, with the proviso that we still reserve the right to make the final judgement whether contributed code is of a sufficient quality and generality to be included in the distributed version of the plug-in. Finally, we emphasize that the PYTHIA collaboration is providing this plug-in without any guarantees, explicit or implied. It is up to the user to cross check that the graphics produced by this plug-in accurately reflect the underlying histogram contents.

Configuration, Compilation, and Linking

To use the LiveDisplays functionality, linking to ROOT must be enabled, using the --with-root=... configuration option. Note also that the LiveDisplays plug-in requires that the linked version of ROOT must be at least v.6+ (e.g., our use of transparency is not backwards compatible with ROOT v.5.) Once you have configured and compiled PYTHIA 8 with these options successfully, you can check whether the plug-in works by seeing if you can compile and execute the example program in PYTHIA's examples/ subdirectory. We note that it is important that your ROOT libraries were compiled with the same C++ compiler version (or at least a compatible one) as the PYTHIA 8 (and LiveDisplays) libraries were. Otherwise you may get errors related to inconsistent architectures. We kindly ask you not to use us as a helpdesk for troubleshooting problems of this kind.

The LiveDisplayApp object

In each main program that uses the LiveDisplays plugin, a single LiveDisplaysApp instance must be created. This takes care of defining the ROOT TApplication instance that handles communication with the computer's graphics display(s), and basic initialisation. Note: only a single LiveDisplaysApp instance should be created per main program.

Creating LiveDisplay objects

We assume that the user has already defined a TH1D (or TH2D) object, which they intend to fill during the run. For example: A run-time display can be created from a TH1D (or TH2D) object, using one of the following three constructors (assuming that one instance of a LiveDisplaysApp has already been created, see above): The 3rd type of constructor allows to show a band defined by the envelope of several TH1D histograms, for instance as obtained by filling separate TH1D histograms with different uncertainty variations, or with different PDF weights, etc. It is then also possible to include an additional pane in the display, underneath the main plot pane, showing the breakdown of relative deviations member by member in the vector although this functionality is currently limited to up to 8 members. This pane can be enabled/disabled by the following flag:

flag  LiveDisplays:showRelUnc   (default = on)
of histograms are present and/or histogram files are read in with multiple weight sets, this flag tells whether one or more additional pane(s) should be added underneath the main plot, showing the relative breakdown of (signed) deviations with respect to the first element of the vector (hence if the first of your weights does not explicitly represent a desired average or reference distribution, be sure to take the appropriate average manually and include it as the first element of the vector). Since the absolute size of the bands can be seen both in the main plot pane and in the ratio pane (if enabled), here only the relative breakdown between the members is shown; i.e., values are expressed in the range [-1, 1], bin by bin, as a fraction of the deviation of the largest (on the positive side) or smallest (on the negative side) member. The second argument is required so that PYTHIA's Settings database can be used to initialise the parameters and flags documented on this page. Note that the values specified in Settings are used as common initialisers for all displays. Howver, settings for each specific display can subsequently be (re)set via appropriate methods. For example, setting LiveDisplays:logY=true (see below) will make the default for all displays be to use a logarithmic scale on the vertical axis. Individual displays can then be switched back to using a linear scale by using the method Display::logY(false). Whenever such a display-specific method exists, the name of that method will be listed, below, together with the relevant Settings parameter. Technical note: the LiveDisplay class inherits from ROOT's TCanvas class. The user can therefore rely on any methods that would usually manipulate or add additional content to instances of the TCanvas class to work equally well for a LiveDisplay. A few basic methods to use as convenient shorthands for adding simple objects such as LaTeX text (via a pointer to a TLatex object) or lines (via a pointer to a TLine object) onto a display are defined below. Further content can be added using any of the methods defined for ROOT's TCanvas class.

Display Size and Placement

mode  LiveDisplays:sizeX   (default = 320; minimum = 32)
Horisontal display size in pixels. A display-specific value for this parameter can be specified as a third, optional, argument to the constructor.

mode  LiveDisplays:sizeY   (default = 320; minimum = 32)
Vertical display size in pixels. Note: when additional panes are shown (e.g., to show ratios) in addition to the main plot pane, this size specifies the total display size including all panes. If multiple panes are shown, it may therefore be useful to choose this size to be larger than the horisontal size. A display-specific value for this parameter can be specified as a fourth, optional, argument to the constructor.

mode  LiveDisplays:offsetX   (default = 16; minimum = 0)
Horisontal display placement in pixels, relative to the left screen edge. A display-specific value for this parameter can be specified as a fifth, optional, argument to the constructor.

mode  LiveDisplays:offsetY   (default = 16; minimum = 0)
Vertical display placement in pixels, relative to the top screen edge. A display-specific value for this parameter can be specified as a sixth, optional, argument to the constructor.

mode  LiveDisplays:cascadeX   (default = 16; minimum = 0)
Horisontal relative displacement in pixels between successive displays. No display-specific value for this parameter can be specified.<

mode  LiveDisplays:cascadeY   (default = 16; minimum = 0)
Vertical relative displacement in pixels between successive displays. No display-specific value for this parameter can be specified.

Main Plot and Axis Ranges

The base TH1D histogram given to each LiveDisplay constructor is used to automatically determine Y and X axis ranges for the displayed plots. The Y axis range is determined by calls to TH1D::GetMinimum() and TH1D::GetMaximum() and should hence be set by calls to TH1D::SetMinimum(value) and TH1D::SetMaximum(value). The X axis range is always the full X range of the histogram. In addition, the following flags and parameters can be used to control the default behaviour of axes and histogram normalisations:

flag  LiveDisplays:logX   (default = off)
displaying X axes on a logarithmic (on) or linear (off) scale. To (re)set for an invidual display, use the method Display::LogX(bool).

flag  LiveDisplays:logY   (default = off)
displaying Y axies on a logarithmic (on) or linear (off) scale. To (re)set for an invidual display, use the method Display::logY(bool).

flag  LiveDisplays:normalize   (default = off)
whether to normalise all displayed histograms to unit area. If a histogram (or file) contains nonzero overflow bins, these will be included when computing the normalisation factor; the displayed area is then 1.-overflow/total. (Note: extension to use the normalisation factor defined in TH1D should be developed.) To (re)set for an invidual display, use the method Display::normalize(bool).

parm  LiveDisplays:normFactor   (default = 1.0)
possibility to multiply histogram contents by an overall factor. To (re)set for an invidual display, use the method Display::normFactor(double).

Titles, SubTitles, Legends, Labels, Etc

Main Plot Title

By default, the main plot title for each display is set equal to the title of the histogram given in the constructor. This can be overridden by a call to Display::title(string). The font size of the title is set by

parm  LiveDisplays:titleSize   (default = 0.047; minimum = 0.01; maximum = 0.1)
Font size (in units of plot size) of main plot title.

Main Histogram Legend Label

A default legend label can be specified for the main histogram (the one used to construct the LiveDisplay object) using

word  LiveDisplays:legendLabel   (default = none)
Fallback label for the main histogram. Can be overriden using the Display::setLegend() method, see below.

Top Left and Right Labels

The following give convenient ways to add simple user-defined labels at the top left and right corners of the main plot plane.

word  LiveDisplays:leftLabel   (default = none)
Label text (in ROOT's TLatex style) placed, if value != "none", above top left corner of main plot, left-justified. Can be modified by Display::leftLabel(string). The properties of the leftLabel may be customised by accessing the member TLatex* Display::leftLabelPtr.

word  LiveDisplays:rightLabel   (default = none)
Label text (in ROOT's TLatex style) placed, if value != "none", above top right corner of main plot, right-justified. Can be modified by Display::rightLabel(string). The properties of the rightLabel may be customised by accessing the member TLatex* Display::rightLabelPtr.

Subtitles (Bottom)

Two lines of subtitle text can be included, with default positions at the bottom of the plot. These can be used, e.g., to provide the reference for an experimental measurement and to provide the list of MC generator versions or other common settings that would be relevant to include on plots.

word  LiveDisplays:SubTitle1   (default = none)
Default string for first subtitle text. Can be modified by Display::subTitle1(string).

parm  LiveDisplays:SubTitle1y   (default = 0.112)
Vertical placement of first subtitle text, with 0 being lower edge of main plot pane and 1 being its top edge. Can be modified by Display::subTitle1y(double).

parm  LiveDisplays:SubTitle1size   (default = 0.033; minimum = 0.01; maximum = 0.5)
Sets the font size of the subtitle text, in units of the plot size. Can be modified by Display::subTitle1size(double).

word  LiveDisplays:SubTitle2   (default = LiveDisplays)
Default string for first subtitle text. Can be modified by Display::subTitle2(string).

parm  LiveDisplays:SubTitle2y   (default = 0.09)
Vertical placement of sceond subtitle text, with 0 being lower edge of main plot pane and 1 being its top edge. Can be modified by Display::subTitle2y(double).

parm  LiveDisplays:SubTitle2size   (default = 0.033; minimum = 0.01; maximum = 0.5)
A scale factor that can be used to increase or decrease the font size of the subtitle text. Can be modified by Display::subTitle2size(double).

Adding a Ratio Pad

flag  LiveDisplays:showRatio   (default = on)
whether ratio pads (e.g., theory/data) should be shown on displays that contain more than one graph. To (re)set for an invidual display, use the method Display::showRatio(bool).

parm  LiveDisplays:ratioMin   (default = 0.5)
ratio plots. To (re)set for an invidual display, use the method Display::ratioMin(double).

parm  LiveDisplays:ratioMax   (default = 1.5)
ratio plots. To (re)set for an invidual display, use the method Display::ratioMax(double).

flag  LiveDisplays:ratioLogY   (default = off)
NOT FULLY IMPLEMENTED YET. Flag telling whether the ratio pane should use a logarithmic (on) or linear (off) Y axis. To (re)set for an invidual display, use the method Display::ratioLogY(bool).

Adding Reference Data Comparisons (e.g., from HepDATA)

A data file can be added to a LiveDisplay object by using the setDataFile() method. A wide variety of column-based formats can be accommodated (with up to twenty columns), for example (assuming a histogram myTH1D has already been created): The first argument specifies the file name of your data file. It is typecast as a string. It should be an ordinary ASCII file with values separated by tabs or spaces. The file may also contain text or lines or other non-numerical content (e.g., furnishing more details about the measurements, etc), but the lines containing the data entries must appear first in the file, as PYTHIA will stop processing data entries after it encounters the first non-numerical line. In the call to setDataFile(), the numerical values following the name of the data file specify the type and order of data columns in the file, column by column. The possible format specifiers are listed here:
Left Bin Edge 10 xLeft 11 errX- 12 1-xRight 13 exp(xL) 14 10xL
Bin Center 20 xCenter 21 xWeighted 22 1-xCenter 23 exp(x) 24 10x 25 ++iBin
Right Bin Edge 30 xRight 31 errX+ 32 1-xLeft 33 exp(xR) 34 10xR
Bin Content 40 y 43 y [logX] 45 k Factor 46 1/k Factor
Stat Errors 50 errStat 51 errStat+ 52 errStat-
Sys/Th Errors 1 (optional) 60 errSys 61 errSys+ 62 errSys-
Sys/Th Errors 2 (optional) 70 2nd errSys 71 2nd errSys+ 72 2nd errSys-
Total Errors (optional) 80 errTot 81 errTot+ 82 errTot-
Merging Info (optional) 90 N
Generic Info (optional, read as strings, ignored by reader) 1 Experiment Name 2-9 Other string-type information
Ignore column 0 Ignore
This gives a large flexibility in the types of data files that can be used, without the user having to reorder content. E.g., the format given in the call to setDataFile() in the example above tells LiveDisplays that this particular data file should be interpreted as having the columns

Rebin MC Like Data

flag  LiveDisplays:RebinLikeData   (default = off)
to on, MC histograms are displayed rebinned using the binning of the reference data histogram. (For displays that do not have reference data, this parameter will be ignored.) Note that the underlying MC histograms are not modified, only their representation on the display. Note also that the binning of the MC histogram should be finer than and ideally compatible with that of the data, so that the effect of the rebinning is purely to collect whole bins into bigger bins. If this is not the case, a very simple (linear) attempt is made to split original bin contents among the rebinned bins, but this will produce distortions in case of non-linear trends. Can be modified by the method Display::rebinLikeData(bool).

flag  LiveDisplays:zeroSkipping   (default = off)
Flag to exclude zeroes when computing averages for rebinning. In some contexts (like average pT vs nCharged), unfilled bins really correspond to undefined values, rather than measurements of zero. In such cases, unfilled entries should be ignored when rebinning; this can be achieved by switching this flag to on. For example, if three bins are to be combined into one but two of them have not been filled yet, the default averaging procedure would be to include those bins so the final result is (w + 0 + 0)/3 = w/3, where w is the contents of the filled bin. When this flag is set to on the rebinned result is w instead of w/3. Can be modified by the method Display::zeroSkipping(bool).

Updating and Saving Displays

Displays can be updated periodically during the event generation by adding a global call to the bool LiveDisplays::update() method. For ordinary applications, just add a call to this method for every processed event (in which case VinciaROOT will count internally how many events have been processed so far) or it can be given an optional argument telling it how far we got, update(int iEvent). To save time, displays will not be refreshed at every call but instead with a frequency controlled by

mode  LiveDisplays:nUpdate   (default = 500; minimum = 1)
The return value of update() will be true if the displays were refreshed in this call, otherwise false.

If desired, the event number at which the displays will next be refreshed by update() can be retrieved with int LiveDisplays::nNextUpdate().

To force an update, an optional boolean argument can be supplied to the method LiveDisplays::update(bool force).

The ROOT example programs included with VINCIA give some further examples of updating.

Saving Displays to Graphics Files

Displays can be automatically saved to graphics files (see paragraph below for how to save histogram data files) as part of the call to update(). To save time, not every call to update() produces a new set of saved displays. The base interval between savings is controlled by the parameter

mode  LiveDisplays:nSave   (default = 2000; minimum = 0)
number at which the displays (and/or histograms) will first be saved. Note: the value 0 switches off histogram/display saving. Since the statistical accuracy scales with (the square root of) the number of processed events, linear intervals in nSave are only used at the beginning of a run. A transition is automatically made to the following form at the point when it produces a longer interval than the linear one:

nSave * pow(2,1/fSave) ,
with the parameter fSave controlling the frequency of saves in the asymptotic part.

parm  LiveDisplays:fSave   (default = 4; minimum = 0.5; maximum = 8)
E.g., a value fSave=1 would correspond to saving the displays each time the statistics have doubled, at which point statistical uncertainties will have been reduced by a factor sqrt(2)~1.4, while the default fSave=4 produces a save each time the statistical uncertainties are reduced by roughly 10% relative to the previous save.

The produced file type(s) are controlled by the following flags

flag  LiveDisplays:saveEPS   (default = off)

flag  LiveDisplays:savePDF   (default = on)

flag  LiveDisplays:saveROOT   (default = off)

flag  LiveDisplays:saveTEX   (default = off)
mode. Note: direct saves to bitmapped file formats like png or jpeg appears to be afflicted by bugs in ROOT. We recommend saving to PDF or EPS formats and manually converting those files as required.

By default, file names for the saved graphics files will be formed from the name of the saved display(s), followed by the appropriate graphics extension (so a display named "Thrust" will be saved to "Thrust.png"). User-specifiable prefixes and/or suffixes can also be provided (e.g., to distinguish plots made in different runs), via the following strings:

word  LiveDisplays:saveFilePrefix  

word  LiveDisplays:saveFileSuffix  

If desired, the event number at which the displays will next be saved by update() can be queried with the method int LiveDisplays::nNextSave().

Saving Histograms to .dat files

It is also possible to save your LiveDisplays histograms to simple ASCII data files, for later plotting with another program or for overplotting in a subsequent VINCIA run. This is controlled by the

flag  LiveDisplays:saveHistograms   (default = on)
Note: the intervals for when to save histogram data files are the same as those for saving graphics files, see above.

By default, the data format is that used by,

If you are running VINCIA with automated uncertainty bands, two additional columns will be added, representing the estimate of the theory uncertainty

The total uncertainty should be computed by adding the statistical and theory uncertainties. It is up to the user to decide whether to add them linearly or in quadrature.

Note: the theory uncertainties are computed from reweighted events, and hence have a slightly larger intrinsic statistical uncertainty than the central y values. This uncertainty is, however, still tightly correlated with the statistical uncertainty on the central result. For the time being, therefore, VINCIA does not output a separate statistical error on the theory uncertainties.

Note 2: in order to preserve total normalisations, VINCIA writes the underflow and overflow bins in the file as well, as the first and last line of the data points, respectively. They are assigned fictitious x values, displaced by one bin from the main bins, so they will not show up on plots restricted to the range of the original histogram. Feel free to edit the files to comment them out or remove them entirely, as suits your purpose, but be aware that by default, the first and last bins represent the under- and overflow bins and are assigned x values displaced from the main histogram range.

Display Options

Optionally, you can set global properties that you wish to use for all displays already at this point, using the keyword "all" in any of the following methods:

flag  LiveDisplays:showChi2   (default = off)
whether to show chi2(5%) values in the plot legend.

  • Optionally, additional histograms can be sent to the same display for (over)-plotting. If you just need to display a single histogram, skip to the next step, else read on. Add (further) histograms to an existing display by using the method: Currently, up to four different histograms can be added to the same display. Note that a ratio pane will automatically be added, showing the ratio of each subsequent histogram to the first one (or to data, if a file with data points is added, see below). In case you don't want to see the ratio pane, see the methods setShowRatio() and setRatioRange() below.
  • By default, VINCIA then automatically adds a ratio pane below the main plot, showing Theory/Data in a +/- 50% window. Note that theory is automatically rebinned in the same way as data in the ratio plot, but to avoid rebinning artifacts it is still important to choose the binning of the theory to be at least as fine as that of the data. The ratio pane can be switched off using the method The name of the experiment and a reference for the analysis can also be read automatically from the data file, if it contains the following two lines I.e., the SRC keyword is used to give the name of the experiment (as it will appear in the plot legend) and the REF one to give the paper reference (as it will appear in the lower part of the plot, above the MC/version names).
  • Optionally, to modify the properties of a single display without modifying the global ones set in step 1, you can use the same methods as in step 1 but inserting the display identifier as the first argument, e.g.,

    where the first variant of the setLegend() method uses an integer to specify the placement of the legend: -1 for left-hand placement, +1 for right-hand placement, and 0 for automatic left/right placement depending on where the data (or first MC) histogram is largest. The second variant of the setLegend() method instead uses absolute coordinates to specify the placement and size of the legend (only the first two arguments are mandatory).

    Note that some properties are so far only modifiable at the global level and cannot (yet) be changed for each individual display.

  • Optionally, the colour palette for each graph on a plot can be set with So far, it is not possible to specify a complete user-defined palette (though this functionality could be added if needed). The predefined colour palettes available in VINCIAROOT were taken from the professional graphic-design literature and are constructed to produce appealing plots with maximum colour contrast under a variety of constraints. In particular, we use the colours from fig 11 in [Zeileis, Hornik and Murrell: "Escaping RGBland: Selecting Colours for Statistical Graphics." Comp. Stat. & Data Analysis 53, Issue 9 (2009) 3259-3270].
  • Optionally, there is some (still very limited) functionality to add additional graphics content to a display. Currently, the only implemented functionality of this type is to add a line created with ROOT's TLine class. You create the line yourself and modify its properties to suit you, then send a pointer to it, to a named display (here called "Thrust") using:
  • Using these steps, you should be able to open runtime displays, send ROOT histograms to them, and define associated data files. All that remains is to update them periodically during the run and/or save them to graphics and/or dat files.

    Plotting Uncertainty Bands

    Optionally, vectors of histograms can be used to represent uncertainty variations. This is especially convenient when using VINCIA's automatic uncertainty estimates, but can in principle also be used for making uncertainty estimates with other Monte Carlo generators. Instead of a single pointer to a TH1D, the display should then be initialised using an object of the type

    The zero ("central") component of this vector will be displayed just as in the case of an ordinary TH1D*, including vertical lines showing the statistical MC uncertainty. However, if myTH1Dvec contains more than one element, VINCIA will also add a shaded box below the point, computed from a simple max and min operation on the uncertainty variations. Note that the size of the shaded box is obtained by summing the max and min variations in quadrature with the statistical MC uncertainty on the central point.

    By default, VINCIA also adds a pane below the main plot showing the relative decomposition of the total variation into its components. A legend for this pane is given in the illustration here to the right, with names corresponding to VINCIA's automatic uncertainties and numbers indicating the vector index. As an aid to visualizing the many possible contributions, blue colours are used for even vector indices (generally corresponding to "High" variations in VINCIA) and red ones for odd indices (generally corresponding to "Low" variations in VINCIA), with the exception of index numbers 1, 8, and 9 which are drawn as black lines.

    The y axis of the relative-uncertainty pane goes from -1 to 1. Above (below) zero, each variation greater than (smaller than) the central one is shown expressed as a fraction of the largest (smallest) variation. Thus, this pane illustrates, bin by bin, which source of uncertainty generates the largest contribution to the total. Due to unitarity, one generally sees a cross-over between reds and blues at the intersection between Sudakov- and hard-radiaton-dominated regions.

    mode  LiveDisplays:optStat   (default = 0)
    Option to include the ROOT optStat information on plots.

    flag  LiveDisplays:verbose   (default = off)
    Option to enable verbose (e.g., debug) output from LiveDisplays.

    Example Programs

    The following example programs are included with the VINCIA distribution and illustrate how to use the ROOT interface: