The SVG's worked flawless for me. ff 1.5 too
Thats a damn interesting chart.
Looks if to shutdown most of the www we just need to make every browser stop rendering on invalid markup... But hey, who didn't know this before?:)

What happens instead of the SVG showing up? Do you get a dialogue window, is it just white, or what?

It's just white... very strange...

I had the same problem, but fixed it after fiddling in the areas given by this Bugzilla comment –
https://bugzilla.mozilla.org/show_bug.cgi?query_format=specific&order=relevance+desc&bug_status=__open__&id=303581#c3

ad html statistics: maybe I have missed something, but I have seen much more complex and imho better presented (wihout SVG usage) survey made by one czech guy
http://www.dgx.cz/tools/tags/cssvault.html – (made on few sites which appeared on CSS vault front page; he made the same with wider range of webpages from czech area ...). I think that he also did a great work.
description [czech only :-(] http://www.dgx.cz/trine/item/statistika-pouzivani-html-znacek/

> I had the same problem, but fixed it after fiddling

Strange, I don't get the "Save as" dialog. I also can't any SVG mime type in my download types options...

I didn't have that mime type until I did the fiddling. I opened a local SVG file, telling Windows to Open With Firefox. Then Firefox popped up the Save as dialog, and I said always open with – some random app. Then I removed the mime type from Firefox, and it all started working.

Ah, that did the job, thanks James.