Crosstab average

I currently have a crosstab showing how many new patients are seen per month/year at each clinic location:

Clinic1 Clinic2 Clinic3 Jan 2015 1 2 1 2016 4 1 2 Feb 2015 2 2 1 2016 1 1 2 March 2015 4 2 1 2016 2 1 2 April 2015 5 2 1 2016 3 1 2 May 2015 1 2 1 2016 4 1 2 June 2015 2 2 1 July 2015 5 2 1

(The value shown for each clinic is a count of a "patient number" field.)

Now, I need to create a 2nd crosstab showing the "yearly average to date" to see if monthly totals are increasing or decreasing. It should look like this:

Clinic1 Clinic2 Clinic3 2015 Avg. 3 2 1 2016 Avg. 3 1 2

I cannot figure out how to create the averages in the 2nd crosstab. Formula field? Other solution?

I also do not know how to deal with the fact that 2015 has 12 months and 2016 has only 5 complete months at this point.

Any help is appreciated!