I'm looking for a charting library that can draw heatmap charts, and has the option of displaying column label text vertically (allowing me to fit lots of columns on the screen regardless of the length of the labels)
Ideally the library would be free for a charity/educational organisation.
Here's an example of a simple heatmap with vertical labels:
That chart was created using FusionCharts, which is not suitable as it is a flash-based product and is expensive to license.
You could always roll this yourself. Most of the table can be generated using ordinary HTML, with the exception of the vertical labels at the bottom. You could tackle that with Rafael, a JavaScript library that allows you to draw on an HTML canvas. Drawing rotated text is actually available out of the box so it won't be hard to implement.
I think what you are looking for is also called a TreeMap (in some circles anyway).
Check out thejit.org
Under the demos checkout the treemap
and the icicle chart
.
At this url you can find what you are looking for heatmapjs