Color and shapes legend

Hi all,

I want to add a legend to my custom chart. It is a map where you can represent 4 dimensions ( axis X, axis Y, color and shape).

So  I create a "classic" legend for my colors :

But what I truly want it's this:

My first question is : how can I change the circle to box or line or triangle ? I can't find where is the parameter to change... I try to change the icon in my renderLegend function :

It didn't change anything...

How can I do this ? Do anyone has done something similar ? Or has any tips on how to do it ?

Thank you !