Line graph with cut off point for average line

Hi. I'm working on an area chart that contains defect counts and a static average of the defect counts. I was able to create a measure that produces a static average line. Now, I want to have the average line cut off to the current month and update when new data is refreshed each month. 

Current measure to get average line: 

Average =

CALCULATE ( COUNTROWS('Table Data'), ALLEXCEPT( 'Calendar','Calendar'[Year] ) )

/[Static Month_number]

   

 

 

Any ideas on how to update my current measure to get an average line that cuts off to the current data? (being sept in this case). Thanks.