Add functionality to existing graph styles

! In this example demonstrate how graph styles can be modified to add further
functionality to them. ... Here we create changesymbol styles based on PyX's
symbol style. For that two additional column names are introduced that way,
that they can be modified in the constructor of the style. The column names are
then registered by the adjusted version of the `columnnames` method. After that
we only need to replace the drawpoint method by a modified version, which
implements the additional functionality. This new created style can than be
used like any other graph style.