# 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
{ref}`hint <working-with-histograms>`.)

:::{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-md} chi2-errors-python-fig
:class: align-center

<img src="../chi2-histogram.png" alt="chi2 histogram">

What I get when I plot chi2 with errors bars turned on.
:::

(making-a-plot)=
::::{admonition} This plot required special handling
:class: 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()`](https://root.cern/doc/master/classTTree.html).

- I learned about `SetTitleOffset` by reading the
  [`TAxis`](https://root.cern/doc/master/classTAxis.html)
  documentation.

- This led me to the list of
  [`TGaxis`](https://root.cern/doc/master/classTGaxis.html) methods.

Yes, it was {ref}`detective work <detective-work>` all over again!

:::{code-block} python
:caption: How to modify a plot automatically generated by ROOT

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()
:::
::::