Split axis at certain values

A typical use-case for a split axis is to use some special positions on the
axis to mark the axis breaks. ... For that the `splitatvalue` function is
available in all places, where expressions are evaluated in the `graph.data`
classes. Here we create a special parametric function where split positions for
the x axis are added by the `splitatvalue` function and the y function is a
simple polynomial of high order. Note that you can set several split positions
by additional parameters of the `splitatvalue` function. In that case
the `splitatvalue` function marks odd regions to be a "None", by which those
data values are ignored by the `splitaxis` instance.
