Linking one axis to another one

When plotting two or more related graphs, you sometimes want to have identical
axes. This example demonstrates how to achieve this goal in PyX using a
`linkedaxis`. ...

In order to arrange the two graphs, we first create a canvas `c`, in which
we insert the first graph `g1` (the lower panel in the output), which contains the graph of a function.

In the next graph `g2` (the upper panel in the output), we now refer to the
x-axis of `g1`. We do this by creating a `graph.axis.linkedaxis` instance
passing the x-axis of `g1` as the only argument. The latter is obtained from
the axis dictionary `axes` of the graph.

Note that for a linked axis, PyX automatically turns off the drawing of the axis
labels, as you can see in the upper panel. If you do not like this behaviour, you
have to pass a different axis painter to the `linkedaxis` constructor.