Using both Category Groups and Series Groups leads to minimum data value of 1

I have a query that gives me a list of all current customers, the software version for each customer, and number of helpdesk calls that each customer has placed in the previous 2 months. 

When I attempt to graph this, using version as the Series Group and number of calls as the Category Group, I'm running into a weird problem: The minimum data value is always 1, even if there were no customers with that version and number of calls.  In the image below, the chart on the left shows what I'm talking about.  The chart on the right doesn't use Series Groups, but instead has a series for each version and then manually filters in the value expression.  The chart on the right is correct.

Image to show value and grouping definition:

Thanks for your help.