i don't know of a way to split those bars but you could add lines in between the bars by adding another Dimension to your Columns Shelf. The easiest one to add is a calculated field named "blank" with the actual field set to " ". Just drag this to the Columns shelf after that first dimension and you will get a nice line in between each bar.
I hope that helps.
Unless someone has a workaround, I believe this is a feature request. I'd recommend creating a new idea in our @ideas section (doing @ideas lets you tag the space) and encourage other users to vote up the idea.
Yuri - now that is cool. Thanks you.
I am marking Yuri's reply correct since this addresses the request.