# How can one use a variable to set color of a line plot , and use colorbar?

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Levente Gellért on 27 May 2024 at 17:24
Commented: Levente Gellért on 28 May 2024 at 8:54
Dear MAtLab community,
I am creating a simple line plot, and I would like to use a third variable to color the line at every point .
the line comes as:
line(koordmean(:,1),koordmean(:,2))
xlim([1 640])
ylim([1 480])
set(gca,'ydir','reverse')
and the line should be colouread according to the variable 'anx'. It should work similar to the imagesc function combined with the colorbar function.
Any suggestions are highly appreciated! Kind regards
lg

DGM on 27 May 2024 at 19:17
Edited: DGM on 27 May 2024 at 19:51
Here's a simpler way to do it using patch().
% the data as given
% the coordinate data
x = koordmean(:,1);
y = koordmean(:,2);
% the color scaling data
c = anx;
n = numel(anx);
CT = parula(n);
% pad the vectors to keep the patch from being filled
x(end+1) = NaN;
y(end+1) = NaN;
c(end+1) = NaN;
% create a patch
hp = patch(x,y,c,'EdgeColor','interp');
hp.LineWidth = 2; % if you want fatter lines
colormap(CT)
colorbar
% if you want to set certain axes properties
xlim([1 640])
ylim([1 480])
set(gca,'ydir','reverse')
Doing it this way only creates one high level graphics object, instead of dozens. That means less lag and there's only one object that you need to edit if you want to change properties after the fact.
DGM on 27 May 2024 at 18:07
Moved: DGM on 27 May 2024 at 19:48
Levente Gellért on 28 May 2024 at 8:54
That's very nice, thank you DGM

Image Analyst on 27 May 2024 at 17:38
If you'd like the line to have a different color at every point along it, you'll have to plot markers with those colors.
markerSize = 10;
numPoints = 500;
cMap = jet(numPoints); % Make colormap
x = 1:numPoints;
y = x; % Sample data.
for k = 1 : numPoints
plot(x(k), y(k), '.', 'Color', cMap(k, :), 'MarkerSize', markerSize);
hold on;
end
grid on;
hold off;
Adjust colormap and markersize as desired.

Austin M. Weber on 27 May 2024 at 18:32
To color the line according the values in the variable anx:
x = koordmean(:,1);
y = koordmean(:,2);
% Normalize the data in anx for color indexing
n = length(x);
anx_normalized = round(rescale(anx, 1, n));
cmap = parula(n);
figure
clim([min(anx) max(anx)])
hold on;
for i = 1 : n-1
% Get color from colormap
color = cmap(anx_normalized(i), :);
% Plot the line
plot([x(i), x(i+1)], [y(i), y(i+1)], 'Color', color, 'LineWidth', 2);
end
hold off
xlim([1 640])
ylim([1 480])
set(gca,'ydir','reverse')
cb=colorbar;
cb.Label.String='anx';
xlabel('koordmean(:,1)')
ylabel('koordmean(:,2)')

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