stackplot_test_baseline has different results on 32-bit and 64-bit platforms

I haven't quite gotten down to the bottom of this one yet, but this is the cause of the current failures on Travis.

On a 64-bit Linux, I get the following for `stackplot_test_baseline` (and I suspect this is correct):

![stackplot_test_baseline-expected](https://f.cloud.github.com/assets/38294/113353/7225c104-6b4e-11e2-84c5-26ce7ea1b4a6.png)

And on 32-bit:

![stackplot_test_baseline](https://f.cloud.github.com/assets/38294/113354/7a4db882-6b4e-11e2-8ccd-cfb3688c3934.png)

The problem appears to be somewhere in the calculation of the data limits.  On 64-bit, the xmin is exactly 0, on 32-bit it's a very small negative number which causes the locator to add an extra step on the left.