I'm using the image generation code I wrote for NOCtool (including a rather hideous, but fairly readable font), it's a sliught win over bit-bashing a GIF back-end, though not much more. The currently included basic operations are "draw line", "draw rectangle (optionally filled)" and "draw text". All operations takes RGB and alpha values (the labels for the day dividers are centred by drawing the text with an alpha of 0.0, checking the return value (the right- most pixel drawn) and halving that, subtracting it from the X offset of the line). At the moment it only has exports to GIF images, via Skippy, but I have considered breaking the code loose and make it intoa free-standing library (more so after finding an alternative use for it).
EDIT: Apparently, something, somewhere is subtly wrong and I end up with broken images. The source of the problem is being tracked down. It was apparently me passing in a zero-length comment and Skippy happily passing that on. Today's graph has been re-built and from tomorrow, there should be fresh graphs.