Fix bugs in stacked histograms

Hi,

This is an attempt to fix both #1679 and #1631, which both came about because of the way that stacked histograms are handled, introduced in #847.

This isn't ready to go in yet, but I'm running into some problems getting it to work correctly, so I thought I'd try to get some more eyes on it.

There are two problems I'm running into:

1. `fill_between` isn't actually filling
2. `fill_between` seems to be screwing up autosetting the y axis limits.

Here is the image I get when I run `test_hist_stacked`. The two histograms are now in the proper order, fixing #1679, you can see the two problems I mentioned. The histograms should be filled and the minimum of the y-axis should be 0.

![hist_stacked](https://f.cloud.github.com/assets/1271014/99857/d5397a56-67ca-11e2-80a2-075c8f6fcbc6.png)

Could someone a little more familiar with `fill_between` take a look at this?