To set the gap between bars in the same group (default 0.618 of the bar width):

chSet 'Gap' 1

The gap is given as the fraction of a bar width. If the barchart is stacked (i.e. there are no groups) this sets the gap between bars. Small negative values are acceptable here, say -0.1 to give slightly overlapping bars. For tower charts it sets the gap between towers, and 0 is the smallest reasonable value.