bar_plot_stacked example unfinished?

[jonathanrocher]

Problem Description
The example in https://github.com/enthought/chaco/blob/master/examples/demo/basic/bar_plot_stacked.py
doesn't do any stacking.

Expected behavior:
I the bars were really stacked, I would expect the sum of all bars to reach (10+5+2) * array([1,2,3,4,5]) (the sum of all values) respectively. Instead, I am getting the following:

Looking at the code, it doesn't use the bar plot's starting_value as expected, so the demo doesn't even seem to try to do the right thing.