Render graphs as SVG instead of PNG
Pros:
1. awesome crispness on Retina displays (currently it’s meh)
2. can be rendered much faster (less server load + better redraw times)
Cons: IE 8 doesn’t support SVG. A possible workaround is using something like Raphaël (a JS library for cross-browser vector graphics), but even a checkbox in the settings "I swear I’m never going to use IE 8, now please give me my SVG already!" would be cool.

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Karlson Pfannschmidt commented
I would extend this by suggesting to always render the graph on the client side using
* SVG for the browser
* Graph libraries on devices:
- Android: http://stackoverflow.com/a/9957335
- iOS: http://stackoverflow.com/q/769749