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I want to overlay two objects of SmoothDensityHistogram using Show, in order to highlight a region of the shared plot space. However, the top object completely obscures the bottom one. I tried to set an opacity for the objects, but SmoothDenstiyHistogram does not accept PlotStyle as an argument to pass either Opacity[...] or Directive[Opacity[...]]. Is there a way to make this work?

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    $\begingroup$ Could you post example code of your histograms? $\endgroup$ – Carl Lange Jan 16 '19 at 20:15
  • $\begingroup$ @CarlLange My histograms are generated from large data files, but any two sets of random data passed into a SmoothDensityHistogram and overlaid will show the issue. $\endgroup$ – avikarto Jan 16 '19 at 20:17
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You can do this by setting the ColorFunction to use Opacity.

Let's define our ColorFunction, which basically just returns a color for a given plot value:

cf[z_] := Opacity[z, RGBColor[z, 1 - z, 1]]

(You can also wrap any of the ColorData functions with that Opacity and it should work)

Now we can use that to show two plots together:

s1 = SmoothDensityHistogram[
  RandomVariate[BinormalDistribution[.2], 10], PlotRange -> {-3, 3}, 
  Frame -> None, ColorFunction -> cf]
s2 = SmoothDensityHistogram[
  RandomVariate[BinormalDistribution[.5], 10], PlotRange -> {-3, 3}, 
  Frame -> None, ColorFunction -> cf]

Overlay[{s1, s2}]

enter image description here

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  • $\begingroup$ Perfect, thanks! $\endgroup$ – avikarto Jan 16 '19 at 20:31
  • $\begingroup$ Do you not want to have something like ColorFunctionScaling -> False? Otherwise, the highest values have the same color even if there's a large difference in the "peakedness" of the two densities? $\endgroup$ – JimB Jan 17 '19 at 4:35
  • $\begingroup$ @JimB That's a good point - probably a good idea to add your own answer working around that issue. $\endgroup$ – Carl Lange Jan 17 '19 at 9:03

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