One quick question, Do you want to show the number of items for that particular point or do you want to show the total number of items in the IN member of the set irrespective of any points being plotted on the graph?
If the first option is your answer then just drag the Number of Records measure from the Measures shelf to the Detail marks card. This will allow you to see the number of records per each point plotted on the graph.
Let me know if you want to do something else.
No, I'm going for the total amount of distinct items in the IN set, if the point is IN (in this case, that number would be 6).