set fixed space between the bars in bachart

How to set a fixed space between the bars and even bargroups, such that the space between them is not altered even though there more number of bars than the allowed screen space.

No files, tests, or entry points are named. Start by locating the bar chart spacing implementation and determine how bar groups currently respond when the available screen space is smaller than the number of bars; done means confirming a fixed gap between bars and groups under that condition.

Written by the indexing model from the issue text.