Combining graphs together to see the bigger picture.

I'd like to be able to see a composite graph of several similar time periods together:

Example 1: Combine all productivity graphs showing hourly data on Tuesdays so that I can see a more rounded picture of when I'm most productive on that day. (I expect it says don't bother trying to do anything but nap straight after lunch!)

Example 2: Stack my bar charts of things tagged "work" each week on top of each other so I can see patterns in the data.