Unable to use average aggregation for CTR data

I’m using DV360 as my data source.

I did add a stacked bar chart to my LookerStudio dashboard, showing device classification in the stacks.

My dimension is “Month”

My metric is “CTR”

Expectation is to showcase average CTR across each stack, but since the CTR metric presumes “Auto Aggregation” instead of showing the average CTR, its being summed.

Please advise how to resolve the above challenge.