The best solution to you query (and yes it is discussed at length) is adding geonext to editor like dragmath. As the focus has been on adding all the math tools to Moodle 2 and tinyMCE we did not bother to publish patches for htmlarea.
However, you can also use asciimathml's asciisvg functionality to simply add graphs using text expressions (no applet.)
While I think a stable working upgrade path to moodle 2 is still sways off, the math tools available will be extensive. In the interim, you may want to change to tinyMCE in moodle 1.9 and roll out the beta of Maunos plugins
If you want to add geonext to htmlarea on your own there are notes in the forum on that and on adding jsxgraph to header or meta files. The tricky bit is purporting a geonext file to the activity so that jsxgraph will parse it. Integration w htmlarea requires parallel tweaks to dragmath (editing two existing files and adding a dialog file)
Mathematics tools
easy way/tool to create graph/chart
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