SVG:::Graph is a pure Ruby library for generating charts, which are a type of graph where the values of one axis are not scalar. SVG::Graph has a verry similar API to the Perl library SVG::TT::Graph, and the resulting charts also look the same. This isn't surprising, because SVG::Graph started as a loose port of SVG::TT::Graph, although the internal code no longer resembles the Perl original at all.
Add subtitle font size to calculation for positioning of subtitle when there's both a title and a subtitle. This helps keep the two from overlapping.