Add a dynamic average line to a line chart visualization

[COG]

I have a line chart with a count of months on the x-axis and a cumulative count custom measure of some metric on the y-axis. There are a number of lines on the chart as a result of a pivot, each one representing a person.

I’d like to add a line representing the average of all the other lines, that is dynamic - i.e. if the chart is filtered, the average will change.

Is it possible to do this?