Currently it is not possible to enumerate all category keys in the Colour Axis in a Combination chart, so it's not easy to set series types (lines/bars) in code - line/bar. In addition, if one tries to do it, Spotfire hangs. If one tries to set the series type by setting the outer most category in a hierarchy (on the axis), the legend updates to show the change, but the Combination Chart itself does not.
Please see this Question/Answer for more details:
https://community.tibco.com/questions/combination-chart-displaying-lines-series#node-695856