```eval_rst
.. _data-theory-comp:
```

Comparing data and theory
-------------------------
For a tutorial on how to do a data-theory comparison, see [here](../tutorials/datthcomp.html).

The name of the data-theory comparison tool is `plot_fancy`. You can
see what parameters in the runcard influence it by typing:
```
validphys --help plot_fancy
```
The basic inputs are a dataset and one or more PDFs. The way a dataset
is to be plotted is controlled by one or more PLOTTING files in the
`commondata` format. These are simple YAML files and ideally each
dataset should have them. It is possible to specify how to transform
the kinematics stored in the commondata, what to use as x-axis or
how to group the plots. The format is described in detail in [Plotting
format specification](plotting-format). The plotting
specifications are supported by small amounts of Python (defining the
various transformations), which are declared in the
`validphys.plotoptions` package.

Note that PLOTTING files are considered part of `nnpdfcpp`, and as
such they are assumed to be correct, so in principle they have no
guarantee of failing early with a good error message. However, you can
set `check_plotting: True` in the input configurations to cause the
PLOTTING files to be processed as soon as the dataset is loaded. This
can be useful while debugging the plotting files, but might cause
a noticeable delay to the startup (due to loading datasets and fktables).
This will warn the user of missing plotting files
and produce nice early error messages if the configuration is not
processed correctly.