I've you're looking to build something custom from primitives and you need ie support I would definitely recommend Raphael.js [http://raphaeljs.com/] They claim ie6+ support and in my own experience this has been true. Granted I was just making little dynamic pie charts, but they looked fantastic in IE6 (I'm actually surprised it isn't in more common usage by designers to certain visual bugs in IE6).
Raphael.js is really just a nice way to handle svg, and so it lacks some of the niceties of d3 as far directly tying your visualization to the data. But it's still a great tool.
That's my current top contender for the switch. I figure we'll start with g.raphael.js and then if we outgrow it we can just upgrade to the full library.
After just having spent a couple days wrestling with g.raphael.js, I would recommend against it. The documentation is vague and incomplete, the generated graphs are inconsistent (e.g., points placed outside of axes), simple bugs with fixes in pull requests have been sitting for months with no response...honestly it looks abandoned.
Raphael.js is really just a nice way to handle svg, and so it lacks some of the niceties of d3 as far directly tying your visualization to the data. But it's still a great tool.