Opacity gradient in custom legend
"to have the blueish edge faded and the redish solid::
You can use a list of colors with desired opacities as option value for "ColorScheme"
. For example,
colorscheme = Append[ColorData["DarkRainbow"]@#, .01 + #] & /@ Subdivide[10];
Graphics[ChartElementData["GradientRectangle",
"ColorScheme" -> colorscheme, "GradientOrigin" -> Left][{{0, 15}, {0, 2}}]]
One can also have a legend which shows colour and opacity separately, as
BarLegend["Rainbow", OpacityFunction -> (# &)]
This is used in built-in functions such as DensityPlot3D
.