thanks for the pointer,
I looked at the sample, but this solution requires intervention during loading script; I think it should be possible to do within chart itself without creating another table and/or column in the table.
Idea was to have dimension Person as x and y axis, and then calculate union based on count (eventID)
I was thinking that I could to that with aggregate (but that appears not to be the case), I tried to use same dimension for X and Y axis (and it creates proper chart but only shows result where I have same x and y value all other fields are null)...
Maybe set analysis... (but I am not proficient enough yet)
(not to mention that my data model is much more complex and would multiply data involved)