Transparency in scatter plot

Say I have two vectors A and B and would like to have a scatter plot of them, however the transparency of each marker must vary according to corresponding values in a third vector C. The transparency for the smallest value of C should be 50% and for the largest value of C should be fully opaque.

Everything I've tried has rendered all points fully transparent as if they are not being plotted at all. Thanks in advance.

Answer 1

[Walter Roberson]

scatter(VectorOfX, VectorOfY, VectorOfPointSizes, VectorOfColor, ...
    'MarkerFaceColor', 'flat', 'MarkerEdgeColor', 'flat', ...
    'AlphaData', VectorOfAlpha)

[Felipe Dicker]

This yields them fully opaque. So far I have

ax1 = nexttile(t_lay);scatter(ax1,RPM(torque>0&power>0),torque(torque>0&power>0),15,'MarkerEdgeColor',[83 40 191]/255/2,'MarkerFaceColor',...
[83 40 191]/255,'LineWidth',1.0,"AlphaData",TPS/100);xlabel(ax1,"RPM");ylabel(ax1,"Wheel Torque (Nm)");

Answer 2

[Star Strider]

It would help to have representative data. I suspect the vector you are using as ‘C’ has very low positive values (perhaps on the order of 0.1 to 0.3 or something similar).

The documentation stares that the transparency vector has to be scaled to be between 0 and 1. This uses the rescale function to accomplish that, creating ‘Cr’ (‘C’ rescaled).

Try this —

N = 50;
A = randn(N,1);
B = randn(N,1);
C = randn(N,1);
Cr = rescale(C,0.5,1);                                  % Scale To Correspond To 'MarkerFaceAlpha' Rnage of (0.5,1)

figure
hs = scatter(A, B, 200, C, 'p', 'filled');
s.AlphaData = Cr;
s.MArkerFaceAlpha = 'flat';
s.MarkerEdgeAlpha = 'flat';
colormap(turbo)
colorbar

Note that ‘C’ works normally to colour the markers, and ‘Cr’ varies their transparency within the allotted range.

EDIT — Corrected typographical errors.

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[Felipe Dicker]

Aren't they all opaque there?

[Frank Gibbons]

That's what I see - no obvious alpha here: none of the overlapping stars shows anything "underneath" it, not even the teal ones, which are middle of the scale.

[Star Strider]

@Felipe Dicker, @Frank Gibbons --

There were typographical errors in my earlier code that I just now caught.

The blue stars should be most transparent, and the red stars should be most opaque. In my corrected code, that is now apparent, especially the two stars at (-1.5,1.5) and the cluster near (-0.4,0.7).

Try this --

N = 100;
A = randn(N,1);
B = randn(N,1);
C = randn(N,1);
Cr = rescale(C,0.0,1);                                  % Scale To Correspond To 'MarkerFaceAlpha' Rnage of (0.5,1)

figure
hs = scatter(A, B, 500, C, 'p', 'filled');
hs.AlphaData = Cr;
hs.MarkerFaceAlpha = 'flat';
hs.MarkerEdgeAlpha = 'flat';
colormap(turbo)
colorbar

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