We actually used this in the U87MG paper to generate our Venn diagrams. Figures are created by adjusting parameters in the URL, though they've added a live chart design tool that makes designing figures a bit easier.
As a simple example, I've been charting my weight loss (yes, I'm on a diet!) using the API:
The API is pretty manual for the time being. For example, axis scaling is completely manual. Notice that I set chd=t:85,75,60,40, which are the weight values (217, 215, 212, 208) relative to the Y-axis scale (which is always ranged 0-100). Also note that to categorize each axis ("Date" and "Weight (lbs)", I have to add a second "x,y" to chxt, then label them in chxl accordingly and center them by adding in surrounding empty sets. Not overly difficult, but definitely manual.
The applications for bioinformatics are pretty huge. First of all, the API just makes some pretty charts easily, so it's a decent choice for figures generally.
For example, here's a Venn diagram of large insertions detected by Breakway in a tumor/germline paired sample from the same patient:
And here's a pie chart showing events detected in the tumor:
These images are linked directly from the API, so check the image location for the code used to generate them.
Probably one of the most powerful parts of the API, though, is the ability to generate them on-the-fly from URLs. This would make it a useful tool for auto-generating figures of performance stats that could be remotely monitored, for example. Could be pretty nice for monitoring sequencer performance, project stats, et cetera.