simple hack to get elastic X to work with binned groups

I don't claim this is correct or general, just want to keep the conversation moving. 

I'd prefer to see a more general solution that also fixes #229. This may just be vocabulary, but I don't see these issues as padding so much as a needed adjustment step because the group domain in general is not what should be used for the axis.  

In other words, bars are not one-dimensional!