Using PlotLegends with a numeric PointSize

Question

Using PlotLegends with a PlotStyle containing (for instance) PointSize[0.02] creates a legend that does not match the plot.

ListPlot[Table[RandomReal[NormalDistribution[], {20, 2}], {2}],
 PlotLegends -> {"a", "b"}, PlotStyle -> PointSize[0.02]]

How can I get a sensible legend when using a number with PointSize? Solutions involving named sizes (like PointSize[Medium]) are inadmissible unless they somehow create points that have a constant size relative to the plot size.

Answer 1

This problem often occurs in plotting and graphics because of the use of Foo instead of AbsoluteFoo for the directives. The former uses sizes relative to the plot size, whereas the latter, as the name suggests, uses absolute sizes.

In the built-in plot legends in version 9, the plot and the legend are actually two separate objects which are just combined, hence PointSize uses the figure size of each of those to compute what 0.02 means. All you need to do is to change it to AbsolutePointSize to keep it consistent between the two. Of course, the argument will also change and AbsolutePointSize[7] looks close enough to what you were gunning for.

Answer 2

In version 10 the PointSize of the legend will automatically match the PointSize of the Plot:

ListPlot[Table[RandomReal[NormalDistribution[], {20, 2}], {2}], 
  PlotLegends -> {"a", "b"}, PlotStyle -> PointSize[0.02]]

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Edit: The answer by @eldo made me realize, that this is only true up to a PointSize that is equal to the default LegendMarkerSize. To get bigger matching PointSizes:

ListPlot[Table[RandomReal[NormalDistribution[], {20, 2}], {2}], 
 PlotStyle -> AbsolutePointSize[15], 
 PlotLegends -> PointLegend[{"a", "b"}, LegendMarkerSize -> 15]]

For further testing you can use:

Manipulate[
 ListPlot[Table[RandomReal[NormalDistribution[], {20, 2}], {2}], 
  PlotStyle -> AbsolutePointSize[ps], 
  PlotLegends -> PointLegend[{"a", "b"}, LegendMarkerSize -> ps]],
 {ps, 1, 50}]

Answer 3

My V10 output doesn't agree with Karsten's answer, f.e.:

ListPlot[Table[RandomReal[NormalDistribution[], {20, 2}], {2}],
 PlotLegends -> {"a", "b"},
 PlotStyle -> PointSize[0.05]]

I tried to fix this by building my own PointLegend:

PointLegend[{
  Directive[Blue, AbsolutePointSize[20]],
  Directive[Orange, AbsolutePointSize[20]]},
 {"a", "b"}]

Unfortunately, above a certain AbsolutePointSize the points don't grow anymore but mutate to rectangles.