MATLAB Color Map defined by segment values

Question

I have a plot in MATLAB that I would like to transform into a colormap (plot shown below). There are several line segments in this plot, and I want each line segment to be colored based on a specific value that is associated with the segment.

For example:

Value of line 1 = 800, plot a specific color

Value of line 2 = 555, plot a specific color ...etc.

Does anyone know how to do this? I have included the part of the code in my program that is making the plots below. In the code I want the color of the line to be dependent on ElementMap(i,6). I don't have a particular preference on the colors as long as I can tell which line segments have a higher value.

Thanks

%% Plot        

for i = 1:length(ElementMap)
    if ElementMap(i,6) < 1000
        x = [ElementMap(i,1);ElementMap(i,3)];
        y = [ElementMap(i,2);ElementMap(i,4)];
        plot(x,y,['-','b','o']);
        hold on;
    end
end

Suever · Accepted Answer · 2016-07-29 18:13:11Z

1

You could determine an indexed color for each unique value in the 6th column and then convert these indexed colors to RGB colors using a colormap of your choosing (here we use parula). Then when plotting each line, specify the Color property.

% Get indices to use for the colormap
[~, ~, ind] = unique(ElementMap(:,6));

% Create a colormap of the correct size
cmap = parula(max(ind));

% Create a color for each plot
colors = ind2rgb(ind, cmap);

% Now plot everything
for k = 1:size(ElementMap, 1)
    x = [ElementMap(k,1);ElementMap(k,3)];
    y = [ElementMap(k,2);ElementMap(k,4)];
    plot(x,y, 'Marker', 'o', 'LineStyle', '-', 'Color', colors(k,:));
    hold on
end

With this approach, the colors won't necessary scale linearly with your data, but each unique value in ElementMap(:,6) will be represented by a different color and smaller values will be differentiated from larger values.

If you don't care about every plot having a unique value, you could do something like the following which would get you a linear mapping between your colors and values.

values = ElementMap(:,6);

% Assign an index to each
ind = gray2ind(mat2gray(values))

% Create the colormap
cmap = parula(numel(unique(inds)));

% Create a color for each plot
colors = ind2rgb(ind, cmap);

% Now plot everything
for k = 1:size(ElementMap, 1)
    x = [ElementMap(k,1);ElementMap(k,3)];
    y = [ElementMap(k,2);ElementMap(k,4)];
    plot(x,y, 'Marker', 'o', 'LineStyle', '-', 'Color', colors(k,:));
    hold on
end

% Now create a colorbar
colorbar()

% Set the range of the colorbar
set(gca, 'CLim', [min(values), max(values)])

edited Jul 29, 2016 at 18:13

answered Jul 29, 2016 at 17:39

Suever

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3 Comments

Grady F. Mathews Iv Over a year ago

Thank you. This is what I was looking for. A followup question: Suppose I want to put a colorbar with the plot. How do make the values on the colorbar correlate to the values of ElementMap(:,6)?

Suever Over a year ago

@GradyF.MathewsIv That would assume that you had a linear mapping between the two which this does not do (unless the values in ElementMap(:,6) are equally spaced.

Suever Over a year ago

@GradyF.MathewsIv I added an example of how you could do this with a colorbar

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