I've been putting together a monitoring app using Rails as the app framework and RRD as the data storage / graphing engine. Early on, I was the one driving a lot of the requirements. Now that the app is in production, I'm getting a lot of reasonable requests that I didn't think of.
One of these is to display some specific data in a pie chart. For some of the statistics I was collecting, a pie chart made much more sense than the standard 1..n dataset in a time series RRD graph. When I first heard the requirement, my instinct was to use one of the well known ruby graphing engines, like Gruff or Scruffy. Scruffy, in particular, looks really useful. However I wanted to get a quick prototype out in front of my users to make sure they really wanted a pie graph, and for that the Google Chart API was the tool for the job. By reducing graph construction to an API call, Google makes it this easy:
basically, I've embedded the following call in an image tag:
While I may end up switching this out for a more full featured server side graphing engine -- the one at PullMonkey looks pretty sweet even if it does require flash -- the point is that I'm leveraging an API to get my app off the ground very quickly. Life is good when app construction is this fast.