Line plot configuration with prediction

Hello,

I have a simple line plot that counts the distinct users by each month. I want to make it so the current month has a line that is dotted to indicate the estimate of projected users based on the average of the current month so far.

For example, I can do a simple plot of distinct users by month and it will be a solid line for all months. I would like to add a partial segment of line that is dotted for only last month to current month that shows the prediction/estimation of distinct users based on daily average in the current month.

How can i do this?