Dynamic Average Line for Column chart based on filter

Hello,

I have a simple calculated table and I would like to have column chart by months, together with an average line across.

The calculated table looks like this:

The column chart looks like this. At the top right hand corner, there is a year_month_order slicer for users to choose how many months of data they would like to see in the chart.

I would like to have an average line across, which is dynamic based on the slicer.
So for example,

- if year_month_no 1 to 19 are selected, the average line will show the total of 19 months divided by 19, which is standard across all months.

- if year_month_no 1 to 7 are selected, the average line will show the total of 7 months divided by 7, standard across all months in the chart.

The chart would look something like this:

Thank you.