Add An Average Line To Identify Trends?

[Shannon-COTR]

I am attempting to track our giving (both overall donation amounts year over year as well as average gift size year over year) and it's easy enough to create a graph for that, but I'd really love to add a dotted average line that averages out the giving across the years.

In other words, I want to be able to see, in general, a line that would show the average donation revenue in the month of January when comparing the last five years, then the average donation revenue for Feb (when comparing the last five years), etc. so that the line will reflect the total average gifts for any given month when comparing multiple years of giving.

My raw data is broken down in the following way:

I can get add an "Average Line" (see below; there are some gaps ) but it just gives me an overall average when what I want is to see the average of each month's giving over the last few years. I hope that makes sense!

[amitchandak]

@Shannon-COTR , Simply add the measure to Y-axis constant line