I have added a custom shape to use in my respository.
I then drag 2 measures into my view. I change both of these into shapes on a line graph.
I select a person shape for one of the measures
I select my custom shape for the other.
I them select "Dual axis"
At this point my custom shape reverts into a square.
If I chose 2 custom shapes for this and select dual axis both of the shapes turn to squares
I am using png file format