Bug when plotting 3 points in scatter MATLAB R2022a

Question

Ethan Welch 2022-9-15

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回答： Walter Roberson 2022-9-15

I have found a bug in the scatter function that I'm not sure how to tackle. If I have a scatter plot with only three elements I get the error: Invalid RGB triplet. Specify a three-element vector of values between 0 and 1. This is because the program thinks I am trying to input an RGB triplet, when I want them to fit within a broader context.

colormap("jet");

X = 1:7;

Y = X;

color_map = X;

scatter(X,Y,45,color_map,"o","filled")

X = 1:3;

Y = X;

color_map = X;

scatter(X,Y,45,color_map,"o","filled")

Warning: Error updating Scatter.

Invalid RGB triplet. Specify a three-element vector of values between 0 and 1.

I can work arround the problem, by plotting each point twice.

X = 1:3;

Y = X;

color_map = X;

if length(X)==3

X = [X,X];

Y = [Y,Y];

color_map = [color_map,color_map];

end

scatter(X,Y,45,color_map,"o","filled")

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Answer 1

Walter Roberson 2022-9-15

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Because an RGB triple is permitted at that place, MATLAB needs to have some code to decide whether you are providing RGB or you are providing a vector with one entry per coordinate. The test for the vector length being 3 (RGB) is done first. But [1 2 3] is not valid RGB because the entries for RGB have to be in the range [0 1]

I think it would be even more confusing if MATLAB looked at the range of values and treated the row vector of length 3 differently depending on whether the values were all in the range [0 1] or not.

Historically this situation did not happen because scatter() required that x and y be column vectors, and was explicit that c had to be a column vector if it was one value per coordinate pair.

X = 1:3;

Y = X;

color_map = X(:);

scatter(X,Y,45,color_map,"o","filled")