Fix bugs related to bottom kwarg in step histograms

This completes the fix to #1763. #1724 partially fixed this, but I just discovered it only solved the problem for the `bar` histtype. This PR fixes it for the `step` histtype also.

I also discovered another small bug while I was doing this, which I fixed. There was some code that should have only been executed for stacked histograms, but was being run for all histograms. My fix doesn't result in any visible change, as the problem was hidden by lines drawing over the top of each other.