Improve the graph widget to handle negative values

We collect statistics on Task statuses, and then display them in a graph. This works great (see attached figure 1).

We also calculate the difference between today's statistics and yesterday's statistics (and this week/last week, this month/last month), and display those beside figure 1. That doesn't work so well (see attached figure 2).

It would be cool if there were a graphing mode that showed positive and negative numbers nicely (see attached figure 3).

In general, more advanced graphing widgets would be fantastic.

thanks, 
-Mark