#11 Make methods virtual

[Steve Lewis]

I looked into the prospect of subclassing plot classes
- looking at LinePlot as a simple sample. All of the
methods in LinePlot are non-virtual. I considered
overriding the class to alter the SuggestXAxis and
SuggestYAxis method and found that there was no way to
do so short of copying the entire class.
I would volunteer to submit patched if the developers
think this is a good idea.