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.
When creating bar graphs, :stack=>:top yields a graph the same as :stack=>:overlap (or not specifying :stack, since :overlap is the default).
I'm working on a fix.