Recently I've been working on a project using Google Charts API and I absolutely love it. It makes pretty graphs and shows data really nicely - and it's very painless. I did trip over a little trick today though which took me a little while to work out, so I thought I'd share it here: how to add two levels of label to an axis.
The axes in Google Charts are a bit interesting, because what they display bears absolutely no resemblance to what data is there - you label the axes separately, even if they are numbers. To label multiple things - in this example a scale and some units, you simply specify the axis more than once. To specify which axes should be shown, use chxt (CHart aXis Title) and set something like this:
Then add the labels as required, for example:
This produces a graph like this:
You can then add labels (using chxl) and ranges (uses chxr) to your axes as you wish - and even add axis labels to the top and right hand graph edges, using the t and r axes, with as many of each as you'd like, and using their list position as a parameter to the range/label settings. I hope this is useful to someone, as it took me a little bit of research to figure it out.