So you want the color legend to be a chart? Can you explain exactly why you need this?
I am using multiple containers (5) in my dashboard and in each container I have 4 graphs which are selected using a parameter via a drop down. If I enable legends for each of my graphs, the RHS of the dashboard gets very cluttered.
I wish to build a legend as a chart so that I can enable the parameter on the sheet so that it appears and disappears using the drop down menu.