I'm looking for a way to create a "range legend" on my chart.

I have what could be considered a heat map in which individual points are colored according to a 3rd value (i.e. neither their X or Y coordinate).  This part seems to be working pretty well and is not the trouble.

However, what I'd like to accomplish is to show a legend on my chart that is a "range legend" (i.e. a legend that shows a gradient with reference points) instead of a legend with discreet values. 

Here's an example of what I'd LIKE to accomplish:



However the best I've been able to do is:


 
I accomplished this by inserting a manual legend with individual cells that have a gradient fill and the labels of the cells are approximations of the value represented by the color - the label placements are NOT exact...

This is just ok but what I'd really like is a) a legend that does this automatically instead of what I've done; and b) be able to annotate the legend with references outside of the gradient at a more precise location.

Is there a better way to create a range legend than what I've done?  Note that I've already checked out the examples and while "All Examples > Chart Gallery > Triangulated Surface > Triangulated Surface" is sort of what I'm looking for, I'd really rather have something slicker like the attached example - the discreet points don't quite give the visual effect I'm looking for.

Thanks!
Bill