Powershell Trendline Line

I have a Bar graph I created from Powershell, and i want to create a "Trendline"  ~I think it may be called.   As shown below, I want a straight line based on a calculated AVG .   I have 100 computers i need worked on for the year,  with 18 weeks remaining in the year,  how many must I complete each week.   

I can't find too much info on it.  I am wondering, do add something like this in my code..    if so, I am sure there is more to it.. 

$workbook.ActiveChart.SeriesCollection(1).Trendlines().Add