Fixed some issues with SVG::Plot that Ronan Oger, the SVG
author, raised - the most interesting one is that now you can pass
options through to SVG by using the svg_options parameter in the
SVG::Plot constructor. Richard suggested that this should have
been in the plot method rather than the constructor, but given that
the existing design has everything in the constructor I thought it was
neater to add the new options there. If it is indeed bad design then
it needs fixing all at once, not confusingly bit by bit.