We created a chart service that lives at chartd.co. It allows you to create responsive, retina-compatible charts with just an img tag. Like this:
That whole chart came from this URL:
We built it a long time ago and StatHat has been using it to include charts in alert and report emails. While we told a few other companies about it, we never officially announced it.
All the documentation for it is on the main page at chartd.co.
Please let us know what you think of it!