se data processing steps are executed before the call to <code>ggplot2</code>
and the identity function is used by default in all of the <code>ggpcp</code> specific layers.
Besides the speed-up by only executing the processing steps once for all layers,
the separation has the additional benefit, that it provides the users with the
possibility to make specific choices at each step in the process. Additionally,
separation allows for a cleaner user interface: parameters affecting the data
preparation process c