Thanks for the suggestion Marcus.
When I do that, the available Classes are:
When I then select Class = IChartProperties, the only available member is MinorGridStyle.
The other classes don't apply.
I'm thinking that dotted or dashed lines are not possible in a scatter chart.
I don't know if dotted or dashed lines are possible within a scatter chart but not all possible settings have an example within the APIGuide and sometimes you could adapt the logic|syntax|parameters from a similar example.
Another way to get hints/ideas to possible parameters could be to export the document layout (from a small dummy app with only the wanted object to reduce the amounts of the output) and then to search within the xml-branches.
But both ways are far away from funny else quite an ugly trial & error approach so that I wouldn't think about it if the aim of it is just only nice to have.