Heat maps with 3 variable

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Huw Wadkin
Huw Wadkin on 14 Jun 2021
Edited: LO on 14 Jun 2021
hello all, I am fairly new to Matlab. What's the best way to create a scatter plot where for example x axis is year, y axis is month and the colour of each point is dependent on average temperature? Like a heat map. Any help is appreciated, I may be missing something obvious. Thanks in advance, Huw
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Scott MacKenzie
Scott MacKenzie on 14 Jun 2021
You can review your options at Types of MATLAB Plots. Problably contour or contourf is what you want.

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Accepted Answer

LO
LO on 14 Jun 2021
Edited: LO on 14 Jun 2021
you can use the function "scatter3", if you have 3 variables (this will display a scatter plot of x and y and z on the 3rd axis). If you use the 3rd variable as color variable (not as z but as "c", see documentation of scatter3) you can color code x and y based on your 3rd var. Use view(2) to flatten the plot and avoid the 3D if you want a heatmap like plot.
S = 5; % size of the dots
% this may or not work depending on the values of Z (you may convert them to fit,
% either multiplying them or rescaling them so that they are > 0 and < 1.
% for example z = z/max(z), this vector will now contain normalized values
scatter3(x,y,zeros(numel(x),1),S,z,'filled','MarkerEdgeColor','none')
view(2)
I alternatively use imagesc (as in this example by Johnathan Epperl), you can adjust the color intensity changing the limits of the color axis (caxis, see documentation). You can visualize a colorbar using the "colorbar" command.
See if it works for your purpose
% Your x-axis
x = linspace(0,2*pi);
% y-axis
y = linspace(0,2*pi);
% a mesh because
[X,Y] = meshgrid(x,y);
% you need data at every possible x-y combination
Z = sin(X).*atan(Y);
% that scales the Z-values and plots a heatmap
imagesc(x,y,Z)
% choose a colormap of your liking
colormap hot
colorbar;
another alternative (assuming x,y,z are your vars)
range_x = max(x)-min(x);
range_y = max(y)-min(y);
average = mean(z);
heatmap = average*ones(100, 100);
for k = 1 : length(x)
X = round( (x(k) - min(x)) * 100 / range_x ) + 1;
Y = round( (y(k) - min(y)) * 100 / range_y ) + 1;
heatmap(Y, X) = z(k);
end
imshow(heatmap,[]);
colormap('hot');
colorbar;

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