Split axes can be used to build axes containing axis breaks.
While split axes work exactly like bar axes (they even share
the same code except for the painter), their usage differs.
A split axes has a tuple as its data value. The first value
of the tuple identifies a sub-axis (for example by an unique
integer) and the second is the value to be passed to the
sub-axis. (For a bar axis its basically the same internally,
but in that case the styles take care of the wrapping of the
data into those tuples.)

Note: Currently the graph styles are not aware of
axis breaks. A generic notification mechanism for
graph styles should be added at a later point.

Another note: Currently there is no support for wrapping data
into tuples except for the splitatvalue function. Its planed
for the future to remove the mathtree (at least in the graph
data handling), so that the creation of a tuple becomes
possible by standard Python constructs.
