I am trying to create a graph that counts the number of entries in 2 tables per month as a moving count of the previous 12 months. Based on this count, I am calculating the average by dividing the count of one table by the other (by adding a formula).

The data I have is for 10 years, but I would like to see only the last 12 months in my graph.

I am attaching 2 graphs. One is with Null values selected. This shows the correct data for the previous 12 months, but has 13-24 months also shown as a null set. One is with Null value not selected. In this case, the data starts tapering off towards to previous 12 months which is wrong.

Would appreciate help on calculating this and displaying it right. 

best regards,

Sony