Exercise 2: Adding error bars to a histogram
(5 minutes)
We’re still plotting the chi2
histogram as a solid curve. Most of
the time, your supervisor will want to see histograms with errors.
Revise the script to draw the histograms with error bars. (Here’s a
hint.)
Warning
The histogram may not be immediately visible, because all the points are squeezed into the left-hand side of the plot. We’ll investigate the reason why in a subsequent exercise.

Figure 37: What I get when I plot chi2 with errors bars turned on.
Note
In case you’re interested, the code below is how I made the above
plot. I learned to use gPad
to access the temporary histogram from
reading the documentation for
TTree::Draw()
. I
learned about SetTitleOffset
by reading the
TAxis
documentation, which led me to the list of
TGaxis
methods.
Yes, it was detective work all over again!
from ROOT import TFile, gROOT, TCanvas, gPad
myFile = TFile("experiment.root")
tree1 = gROOT.FindObject("tree1")
my_canvas = TCanvas()
tree1.Draw("chi2","","e")
# Get the temporary histogram used by TTree::Draw()
htemp = gPad.GetPrimitive("htemp")
htemp.GetXaxis().SetTitle("chi2")
htemp.GetYaxis().SetTitle("number of events")
htemp.GetYaxis().SetTitleOffset(1.5)
my_canvas.Draw()