I've just finished some work, requiring charts. After some research, I found that the only free 'complete' tool for html charts is the Google Charts API. A neat Javascript library, which enables you to create charts - Basically you create a data table structure, add some customization options, and the API generates the chart as SVG markup, and 'plants' it as an iframe inside the designated DOM element.
You can also hook some event handling, there are value tips, and lots of customization options.
Not to be confused with the Google Charts Image API, which is a little different:
The Chart Image API is generated via a GET/ POST URL request from Google servers, and is an image. It has more formatting options and better graphics, but is not interactive, and transfers your data via Google servers, something I am not too happy to do.
Though not everything is perfect with the Charts API, and there are few missing features (The ability to have 'zones' in a line graph, the ability to have more than one line in a combo chart etc.), it is good enough for most uses, and actually goes in line with the simpler look web applications are taking today.
So the next time you consider spending money on interactive charts, consider these tools. They will look better on your iPad, that I promise.
