"unknown" labels in bar chart

I'm trying to create a View that will show me a bar chart of the number of user accounts that were created each month. I have installed the "Views Date Format SQL" module to be able to create the counts correctly. I create a view with the following:

UID
Created Date - with a custom format "Y-m" - and to use SQL to generate the string

I then turn on aggregation, I set the "Created Date" field to group, and the UID field to count. I then get a table with the number of accounts created by month, where the months are shown as : 2012-06, 2012-07, etc.

I then change the view to type Chart, with the UID field as the Chart Series, the Created Date as the label, and the Google Chart engine. The bar chart shows up correctly, with the height of the bars as expected, but the label under every bar is "unknown".