Adding average line in Line-Chart?

[Rachel Sleeps]

Hi, I recently asked a question (here) - which I got a great answer for. I got a related answer today: I want to add an average line to an ensemble of line in one chart. Here is what my chart looks like so far:

It is a nice little line chart I created from sales data of several stores over time. I am trying to create an "average store" so to speak. How would I go about to do that? Any ideas? Sofar, I experimented with collapse, but that only brought me to a single "average" line.

Best / R

[Nick Cox]

Calculate something like

egen average_sales = mean(monthly_sales_by_size), by(year_month)

and then add as an extra line.

[Rachel Sleeps]

Ah, perfect! Thank you! I didn't know it's possible to add several lines at once. The help function could have brought me here - but you did, Nick! Thank you!