rather simple - in a barchart, you have bars (or columns) and in a line chart, you have lines.
In a barchart, segments (expressions) can be either grouped (side by side) or stacked (on on top of another), but not both - mixing grouping and stacking can only be done in a
=> combochart, that sort of combines bar- and line-chart, except you cannot stack expressions - unless you introduce a second artificial dimension - then those segments which you want side by side must be on different "sides" of that auxiliary dimension, all segments "on one side of it" will be stacked.
