SSRS: Line Chart, Grouping by Day

I have a fairly specific set of data I am trying to display.

I have a dataset of helpdesk ticket data. I would like to display a line chart that shows tickets opened and tickets closed by day, over the last 30 days. I've been able to display one of these just fine, by adding an expression to my Category group:

=Day(Fields!Created.Value)

The 'Created' line displays just fine, yet the 'Closed' line seems to jump around at random. It seems that I can't further group by day on the 'Closed' column; this breaks the chart even further. Here's an example image:

Please let me know if I haven't been clear in any way.