Sorting by Month-Year in Chart

I need to produce a chart of the number of issues reported monthly. The database contains a DateTime field in "MM/DD/YYYY HH:MM:SS" format recording when the issue was entered into the database. Therefore, I am pulling out the Month and Year from the DateTime field. I am having problems with the data sorting in chronological order within chart, whether I use character strings or integers for the "On change of". I have concatenated the Month/Year fields and used the resulting field. I have also created groups for year and month, but I still cannot control the sort order to be chronological. Any help would be greatly appreciated.

Thanks.

Bill