(rdf-make-histogram)=

# Walkthrough: Making Histograms
**(10 minutes)**

This tutorial is all about making histograms, so let's use
`RDataFrame` to make a histogram. Here's an example of making a
histogram of `chi2` from the dataframe:

:::{code-block} c++
:name: cpp-rdf-hist
:caption: RDataFrame - create a histogram (C++)
histchi2 = dataframe.Histo1D("chi2");
:::

:::{code-block} python
:name: python-rdf-hist
:caption: RDataFrame - create a histogram (Python)
histchi2 = dataframe.Histo1D("chi2")
:::

We've made the histogram. Now let's draw it:

:::{code-block} c++
:name: cpp-rdf-draw
:caption: RDataFrame - draw a histogram (C++)
histchi2->Draw();
:::

:::{code-block} python
:name: python-rdf-draw
:caption: RDataframe - draw a histogram (Python)
histchi2.Draw()
:::

Give it a try!

What's that? It didn't work? It didn't if you're using a notebook. 

I'm being sneaky: I counted on you to forget the bit about needing a
canvas (either in {ref}`C++ <ckernel>` or {ref}`Python
<first-notebook>`). Here's a more complete example:

:::{code-block} c++
:name: cpp-rdf-canvas
:caption: RDataFrame - drawing a histogram with a canvas (C++)
TCanvas canvas;
histchi2->Draw();
canvas.Draw();
:::

:::{code-block} python
:name: python-rdf-canvas
:caption: RDataFrame - drawing a histogram with a canvas (Python)
canvas = ROOT.TCanvas()
histchi2.Draw()
canvas.Draw()
:::

:::{tip}
While we have to explicitly Draw a canvas, we can re-use a
previously-defined canvas (the same way command-line ROOT keeps
re-using `c1`).
:::

Follow the above examples to make histograms of the other variables in
the n-tuple.

:::{hint}
I will permit you to use copy-and-paste (in a notebook) or the up-arrow
key (if you're using the command line) to speed up this task. The fact
that I have no way to stop you has nothing to do with it. 
:::

:::{figure-md} flawed_data-fig
:align: center

<img src="https://imgs.xkcd.com/comics/flawed_data.png" alt="xkcd flawed_data" width="75%">

<https://xkcd.com/2494/> by Randall Munroe
:::