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Automatic TLegend positioning



Since I learned that there is a TPad::BuildLegend I was disappointed that it does not automatically try to find a good place.

Here is some Python code that does just that:

import ROOT
import itertools.chain

# Some convenience function to easily iterate over the parts of the collections

# Needed if importing this script from another script in case TMultiGraphs are used

# Start a bit right of the Yaxis and above the Xaxis to not overlap with the ticks
start, stop = 0.13, 0.89
x_width, y_width = 0.3, 0.2
PLACES = [(start, stop - y_width, start + x_width, stop),  # top left opt
          (start, start, start + x_width, start + y_width),  # bottom left opt
          (stop - x_width, stop - y_width, stop, stop),  # top right opt
          (stop - x_width, start, stop, start + y_width),  # bottom right opt
          (stop - x_width, 0.5 - y_width / 2, stop, 0.5 + y_width / 2),  # right
          (start, 0.5 - y_width / 2, start + x_width, 0.5 + y_width / 2)]  # left

def transform_to_user(canvas, x1, y1, x2, y2):
    Transforms from Pad coordinates to User coordinates.

    This can probably be replaced by using the built-in conversion commands.
    xstart = canvas.GetX1()
    xlength = canvas.GetX2() - xstart
    xlow = xlength * x1 + xstart
    xhigh = xlength * x2 + xstart
    if canvas.GetLogx():
        xlow = 10**xlow
        xhigh = 10**xhigh

    ystart = canvas.GetY1()
    ylength = canvas.GetY2() - ystart
    ylow = ylength * y1 + ystart
    yhigh = ylength * y2 + ystart
    if canvas.GetLogy():
        ylow = 10**ylow
        yhigh = 10**yhigh

    return xlow, ylow, xhigh, yhigh

def overlap_h(hist, x1, y1, x2, y2):
    xlow = hist.FindFixBin(x1)
    xhigh = hist.FindFixBin(x2)
    for i in range(xlow, xhigh + 1):
        val = hist.GetBinContent(i)
        # Values
        if y1 <= val <= y2:
            return True
        # Errors
        if val + hist.GetBinErrorUp(i) > y1 and val - hist.GetBinErrorLow(i) < y2:
            # print "Overlap with histo", hist.GetName(), "at bin", i
            return True
    return False

def overlap_rect(rect1, rect2):
    """Do the two rectangles overlap?"""
    if rect1[0] > rect2[2] or rect1[2] < rect2[0]:
        return False
    if rect1[1] > rect2[3] or rect1[3] < rect2[1]:
        return False
    return True

def overlap_g(graph, x1, y1, x2, y2):
    x_values = list(graph.GetX())
    y_values = list(graph.GetY())
    x_err = list(graph.GetEX()) or [0] * len(x_values)
    y_err = list(graph.GetEY()) or [0] * len(y_values)

    for x, ex, y, ey in zip(x_values, x_err, y_values, y_err):
        # Could maybe be less conservative
        if overlap_rect((x1, y1, x2, y2), (x - ex, y - ey, x + ex, y + ey)):
            # print "Overlap with graph", graph.GetName(), "at point", (x, y)
            return True
    return False

def place_legend(canvas, x1=None, y1=None, x2=None, y2=None, header="", option="LP"):
    # If position is specified, use that
    if all(x is not None for x in (x1, x2, y1, y2)):
        return canvas.BuildLegend(x1, y1, x2, y2, header, option)

    # Make sure all objects are correctly registered

    # Build a list of objects to check for overlaps
    objects = []
    for x in canvas.GetListOfPrimitives():
        if isinstance(x, ROOT.TH1) or isinstance(x, ROOT.TGraph):
        elif isinstance(x, ROOT.THStack) or isinstance(x, ROOT.TMultiGraph):

    for place in PLACES:
        place_user = canvas.PadtoU(*place)
        # Make sure there are no overlaps
        if any(obj.Overlap(*place_user) for obj in objects):
        return canvas.BuildLegend(place[0], place[1], place[2], place[3], header, option)
    # As a fallback, use the default values, taken from TCanvas::BuildLegend
    return canvas.BuildLegend(0.5, 0.67, 0.88, 0.88, header, option)

# Monkey patch ROOT objects to make it all work
ROOT.THStack.__iter__ = lambda self: iter(self.GetHists())
ROOT.TMultiGraph.__iter__ = lambda self: iter(self.GetListOfGraphs())
ROOT.TH1.Overlap = overlap_h
ROOT.TGraph.Overlap = overlap_g
ROOT.TPad.PadtoU = transform_to_user
ROOT.TPad.PlaceLegend = place_legend

Obviously this should be implemented in the actual C++ source, then the monkey-patching at the end would no longer be necessary.

After saving this to a script, e.g. legend.py you can just do:

from legend import *
c = ROOT.TCanvas()
h = ROOT.TH1D("h", "h", 100, 0, 100)

Note that I had some more places to try originally, but left them out for brevity. One can even try to calculate an optimum size for it (using the number of entries and the longest title).



Look here: https://sft.its.cern.ch/jira/browse/ROOT-8128 I.e. it’s in the works! Once we have it we’d like to get your feedback - your algorithm is a good start, and I’d hope we will end up with something even better :wink:




Thans for your input. Yes that a good start. And I will look at it very closely. Many thanks.


Am I right thinking that your algorithm tries to find among 6 predefined positions which one does not overlap any object in the canvas and does not try anything else to adjust the legend position ?


Yes, that is what the posted code does.

However, it is not too difficult to come up with a way to try to guess a good size. For example I use this, which uses some heuristic magic numbers to go from the length of the longest entry to a good sized legend:

def get_optimal_size(objects):
    title_len = max(len(x.GetTitle()) for x in objects)
    entry_len = len(objects)
    return 0.007 + 0.023 * title_len, 0.06 * entry_len

and then do this in place_legend:

x_width, y_width = get_optimal_size(objects)
PLACES = [(start, stop - y_width, start + x_width, stop),  # top left opt
          (start, start, start + x_width, start + y_width),  # bottom left opt
          (stop - x_width, stop - y_width, stop, stop),  # top right opt
          (stop - x_width, start, stop, start + y_width),  # bottom right opt
          (stop - x_width, 0.5 - y_width / 2, stop, 0.5 + y_width / 2),  # right
          (start, 0.5 - y_width / 2, start + x_width, 0.5 + y_width / 2)]  # left

I also tried writing a loop which starts from this optimal size and makes it smaller until it fits somewhere, but then you need to define a good cut-off below which it does not make sense anymore to shrink further and just use the fall-back.

Note that this function only works for one-column legends…


The magic numbers I got by dumping legends with different lengths of entries and seeing where the font size changed to one font size smaller. This can probably be done in a better, more consistent, programmatic way.


In the end, what the idea of the code is, is a function that can take places and a canvas and check if there are any overlaps with these places in that canvas. This is one of the necessary steps for good automatic legend positioning. The other ones are knowing where to place it and with which size. The latter one can solve with the second function I posted or one similar to it, for the former I would say a few hard-coded places are good enough.

But one could of course write a grid search function that tries many different places. Or even make a fine grid on the canvas and mark cells in this grid as dirty if any object crosses it and then find the largest rectangular region of not dirty cells. All of this was more complicated than what I needed, though, for my legends usually have similar number of entries and no overlong entries, so one size fits (almost) all.


Thanks for the additional details.