Detailed Description

Create and draw two graphs with error bars, superposed on the same canvas

We first draw an empty frame with the axes, then draw the graphs on top of it Note that the graphs should have the same or very close ranges (in both axis), otherwise they may not be visible in the frame.

Alternatively, an automatic axis scaling can be achieved via a TMultiGraph

See the TGraphErrors documentation

 
import numpy as np
import ROOT
 
c1 = ROOT.TCanvas("c1", "2 graphs with errors", 200, 10, 700, 500)
c1.SetGrid()
 
# draw a frame to define the range
hr = c1.DrawFrame(-0.4, 0, 1.2, 12)
hr.SetXTitle("X title")
hr.SetYTitle("Y title")
c1.GetFrame().SetBorderSize(12)
 
# create first graph
# We will use the constructor requiring: the number of points, arrays containing the x-and y-axis values, and arrays with the x- andy-axis errors
n1 = 10
xval1 = np.array([-0.22, 0.05, 0.25, 0.35, 0.5, 0.61, 0.7, 0.85, 0.89, 0.95])
yval1 = np.array([1, 2.9, 5.6, 7.4, 9, 9.6, 8.7, 6.3, 4.5, 1])
ex1 = np.array([0.05, 0.1, 0.07, 0.07, 0.04, 0.05, 0.06, 0.07, 0.08, 0.05])
ey1 = np.array([0.8, 0.7, 0.6, 0.5, 0.4, 0.4, 0.5, 0.6, 0.7, 0.8])
# If all x-axis errors should zero, just provide a single 0 in place of ex1
gr1 = ROOT.TGraphErrors(n1, xval1, yval1, ex1, ey1)
gr1.SetMarkerColor(ROOT.kBlue)
gr1.SetMarkerStyle(21)
# Since we already have a frame in the canvas, we draw the graph without the option "A" (which draws axes for this graph)
gr1.Draw("LP")
 
# create second graph
n2 = 10
xval2 = np.array([-0.28, 0.005, 0.19, 0.29, 0.45, 0.56, 0.65, 0.80, 0.90, 1.01])
yval2 = np.array([0.82, 3.86, 7, 9, 10, 10.55, 9.64, 7.26, 5.42, 2])
ex2 = np.array([0.04, 0.12, 0.08, 0.06, 0.05, 0.04, 0.07, 0.06, 0.08, 0.04])
ey2 = np.array([0.6, 0.8, 0.7, 0.4, 0.3, 0.3, 0.4, 0.5, 0.6, 0.7])
gr2 = ROOT.TGraphErrors(n2, xval2, yval2, ex2, ey2)
gr2.SetMarkerColor(ROOT.kRed)
gr2.SetMarkerStyle(20)
gr2.Draw("LP")

Author: Rene Brun, Jamie Gooding

Definition in file gr003_errors2.py.