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MathJax doesn't (yet) have support for any diagram package (like xypic or tikz).
If you're talking about a picture diagram, I think the easiest thing to do is to take a screenshot, upload the image to some free image hosting site and include a link (as I did in this question).
If you're talking about a commutative diagram, I usually make the diagram in an array
(as in this question). If you need to draw a more complicated commutative diagram with bending arrows and other fanciness, it's probably easiest to resort to the screenshot method.
Are there any plans to ever include support for a diagram package in MathJax? It seems extremely difficult without server side rendering. Perhaps one day we could have an "extended math mode" on MO that did the rendering server side.
When we contacted the MathJax folk (just over a year ago), it sounded like implementing some subset of xypic was in the long-term plan. I don't know of any concrete plans yet, but I'm not especially in the know. There are certainly bigger fish to fry first, like implementing a cache.
This is not strictly on topic. I just learned about JSXGraph, a javascript diagram library. The examples look fantastic, and it looks like it's extremely easy to make them interactive (I think they might actually be interactive by default). This is off-topic for two reasons:
... or you could automatically convert your TikZ diagram to SVG and include it as it is with no javascript trickery. (Does MO support SVG? Somehow, I doubt it; so that remark is for bloggers as well.)
Any news/interest in representing commutative diagrams in MathJax? It's not the biggest deal to use one of the workarounds described above, but it would be really nice to generate diagrams directly the MathOverflow prompt.
Tom: MathJax 2.2 beta has support for basic commutative diagrams (no diagonal, curved, etc. arrows) via the AMScd package, and seeing as how it is now in use on math.SE, I imagine it will also be available on MO after the move to the SE network.
Zev, 2.2 is not in beta anymore.
In response to Anton's comment, I try never to post links whose future I can't control. If Anton's free hosting service goes out of business, his diagrams will disappear and his postings will be unreadable. Much better, I think, to buy a domain name, host it wherever you like, and point your links to that domain name. Then if your hosting service disappears, you can move your content elsewhere and your links will still work.
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