Unable to chart groups in Web Report Writer

We're using SAM version 6.2.3. We have a custom field in groups for "group_type". Ideally I would like to create a Pie chart displaying all the groups, divided by group type. But I'm having a problem...

So first I start a new report, select Custom Table, and using dynamic query select all of the groups.

Next I select the Group_Type field as a column and Group Name.

I preview the resource and the table looks fine.

Now I use the "Add Content" button to add a "Custom Chart".

I select the same datasource I used for the table.

When prompted to select a series for the X axis in the field-chooser pop-up I only have one option; "Group Availability" which has a sample value of 100.

So my question is why do I see the fields I need in the Table section and not in the Chart section?

Thanks.