So I haven't worked with the Stacked Column Bar Chart, but I'd imagine it has a method similar to the PieChart's (PieDataSet rather) 'setSliceSpace()', which allows spacing in between slices in a PieChart.
This call is in the dataSet itself, I used this tutorial for a PieChart when I was creating it; https://www.youtube.com/watch?v=VfLop_oLYU0&index=12&list=PLFzy3mLpWqF2-E-1bzaAzqh6WgkplbWs1
You can see this method at 11:25 in the video, right after he creates the DataSet.
I'm not sure what kind of DataSet a Stacked Column Bar Chart uses, but hopefully that's helpful!
I've implemented something similar in an app I'm working on (PieChart), and I've played around with this setting (anywhere from 3-10dp/px) to see what value makes sense and looks good on the PieChart.