Generate average line chart over past years data

Hi,
I have a series showing gas consumption values by day for the past 5+ years. I am charting this data aggregated for each month:

I would like to add a “shadow” series (line) to the chart that would show the average value of a given month based on the past 5+ years worth of data to compare current value against the long-term average.

How could I generate this average series?