Drawing an error band graph in PyROOT

I just spent a long time trying to work out how to draw an error band graph in ROOT without things looking completely ridiculous. Often ROOT will mess things up with Draw(“l3”) if you have data points that lie outside the range being plotted. To save others the major hassle involved, I’ve put my custom function here.

This function takes a TGraph or a TGraphAsymmErrors as an input and then draws the error band plot. Feel free to edit it for your own purposes of course. It enables you to create plots like:


An example of a plot using the DrawErrorBand() PyROOT function.

def DrawErrorBand(graph):
    isErrorBand = graph.GetErrorYhigh(0) != -1 and graph.GetErrorYlow(0) != -1
    npoints     = graph.GetN()

    if not isErrorBand:
        graph.Draw("l same")

    # Declare individual TGraph objects used in drawing error band
    central, min, max = ROOT.TGraph(), ROOT.TGraph(), ROOT.TGraph()
    shapes = []
    for i in range((npoints-1)*4):

    # Set ownership of TGraph objects
    ROOT.SetOwnership(central, False)
    ROOT.SetOwnership(    min, False)
    ROOT.SetOwnership(    max, False)
    for shape in shapes:
        ROOT.SetOwnership(shape, False)

    # Get data points from TGraphAsymmErrors
    x, y, ymin, ymax = [], [], [], []
    for i in range(npoints):
        tmpX, tmpY = ROOT.Double(0), ROOT.Double(0)
        graph.GetPoint(i, tmpX, tmpY)
        ymin.append(tmpY - graph.GetErrorYlow(i))
        ymax.append(tmpY + graph.GetErrorYhigh(i))

    # Fill central, min and max graphs
    for i in range(npoints):
        central.SetPoint(i, x[i], y[i])
	min.SetPoint(i, x[i], ymin[i])
	max.SetPoint(i, x[i], ymax[i])

    # Fill shapes which will be shaded to create the error band
    for i in range(npoints-1):
        for version in range(4):
            shapes[i+(npoints-1)*version].SetPoint((version+0)%4, x[i],   ymax[i])
            shapes[i+(npoints-1)*version].SetPoint((version+1)%4, x[i+1], ymax[i+1])
            shapes[i+(npoints-1)*version].SetPoint((version+2)%4, x[i+1], ymin[i+1])
            shapes[i+(npoints-1)*version].SetPoint((version+3)%4, x[i],   ymin[i])

    # Set attributes to those of input graph
    for shape in shapes:

    # Draw
    for shape in shapes:
        shape.Draw("f same")
    min.Draw("l same")
    max.Draw("l same")
    central.Draw("l same")

3 thoughts on “Drawing an error band graph in PyROOT

  1. Alkass

    Hi! This is a nice code, I like it 😉 What the input of that should be ? Can it work with a plain TH1F?


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