How to Create a Color Gradient Legend for Custom Shape Map

Hi Community members,

I have created a custom shape map of a state county boundary. I am using the map to show the county-specific values of some numeric fields (i.e., % maltreat, a created measure based on two other fields). I added the field "% maltreat" to Color Saturtion and the map displays the color gradients based on the values of each county: higher values corresponds to darker colors. See screenshot below.

I woul like to add a simple-bar legend with the color gradient (and possibly min and max values). However, it seems the custom map does not have the function to do that. When I put "county" or other fields in the legend field, each county/value has a separate color displayed instead of a gradient color.

I know that the map tool in Excel will automatically generate a color gradient bar/legend. Is there anyway the custom shape map can do this? Or is there any custom visual for creating color gradients bar? I checked online and it seems there is no easy solution for this, probably because the outcome data is dynamic. Any advice would be appreciated.