Adding a projected outcome (dotted line) to a Google Sheets chart.

Hello all,

I am looking for a way add a (dotted line) projected value into a Google Sheet chart. I am plotting two sets of numbers on a graph, but the final averages for January are incomplete, and therefore show a misleading result. The averages in columns B and C are pulled from collected data in columns P ->.

The projected value could be either calculated automatically, and based on the total number of reported values in time elapsed. Meaning that the value will continue to go up. The value equals the number of days past, up to a maximum of 50.

If automating the projection is too complicated, I will also accept inputting manual value. I can work out my own projection analysis later. Right now, I am more interested in knowing how to create a chart like my mockup; see sheet.

In the example: https://docs.google.com/spreadsheets/d/1Q2nN-MVD_iwJi2Se-PFWcNbMGb-EF6gQWQCruhnWW-Q/edit#gid=180731546
See chart experiment tab.