This method is used to set the min height (width) in pixels for each group in the parts for vertical (horizontal) orientation.
The example on the left is using the default min which is 0 and the example on the right has min 200 pixels. As a result the blue and orange bars have white space added so that it occupis 200 pixels.
The min parameter is useful to stop labels from overlapping when their values are too small by ensuring that there is a min size for each group.
For other options and examples, check VizJS