color scale legend

for scatter plots, conditional formatting, and line plots, enable data viz to have a legend that is a color scale gradient

for example,
1) in scatter plot using saturation, we should have a legend that indicates what the darker and lighter colors represent
2) same goes for conditional formatting in tables and matrices
3) line charts without a field in the "legend" bucket (i.e. line charts represented by a single line) should be eligible for a line graph that has its color change according to a gradient as the value increases or decreases (e.g. a line graph that plots a growth rate could have green as positive growth and red as negative growth)

What's key here is that the legend shows the gradient, which means the max, min, and perhaps a few other points (e.g. middle) would be called out with their numeric value in the legend.