# Exercise 3: Displaying fit parameters 
**(10 minutes)**

Fit the **`ebeam`** histogram to a Gaussian distribution.

:::{note}
OK, that part was {ref}`easy <fitting-histogram>`. It was particularly easy because the "gaus"
function is built into ROOT, so you don't have to worry about a
user-defined function.
:::

Let's make it a bit harder: the parameters from the fit are displayed
in the ROOT text window; your task is to put them on the histogram as
well. You want to see the parameter names, the values of the parameters,
and the errors on the parameters as part of the plot.

:::{note}
This is trickier, because you have to hunt for the answer on the ROOT
web site... and when you see the answer, you may be tempted to change
it instead of typing in what's on the web site (though in Python you'll need either a prefix of `ROOT.` or
including it on the {command}`from import` line).

Take a look at the description of the `TH1::Draw()` method. In that
description, it says "See THistPainter::Paint for a description of all
the drawing options." Click on the word "THistPainter". There's lots of
interesting stuff here, but for now focus on the section "Fit
Statistics." (This is the same procedure for figuring out the "surf1"
option for {ref}`Exercise 1 <detective-work>`).

There was another way to figure this out, and maybe you tried it: Draw a
histogram, select {menuselection}`Options --> Fit Parameters`, fit a function to the
histogram, save it as c1.C, and look at the file. OK, the command is
there, mingled with the `TPaveStats` options... but would you have
been able to guess which one it was if you hadn't looked it up on the
web site?
:::

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

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

<https://xkcd.com/2579/> by Randall Munroe. This is why physicists are never abducted by aliens: There'd
be too much that the aliens would have to look up on their web site.
:::