Although I've never tried the technique you use here to create a single image with php only, wouldn't it be better and easier to use html and some css to set the height of the bars for example?? It would be more accessible since people can still view the data because the numbers are displayed as html text instead of an image.

Thanks for the tutorial though, I'm not that experienced with php and might use this to add some text to thumbnail images. I'm wondering if it's possible to adjust the opacity of a certain rectangle you're creating for example, any ideas?