I use a lot of charts that show the monthly count and then a running average over time. Sometimes, I'm not getting the option to do the calculation over time. 

What is the data table requirement to allow calculations over time?

Here is the table and the chart I'm trying to create. I'm attempting a combo chart, the bars showing the Total Count for that month and then a line for the running average. When I attempt to use the average running total, the option is not showing on the drop down list.