Hi Phil and thanks for your interest in the Questionnaire module and its "personality test" feature with graphs.
At the moment I have selected a small number of graph types which I thought were the more relevant to the analysis of the "personality test" feature. Extra graph types (provided by the RGraph libraries) are not "pluggable", so adding new types would mean significant development.
However, I'd be interested to hear in what contexts you would find scatter and bubble charts useful. You can contact me directly at my email address in my profile.
Joseph