How To Label Colormaps in MATLAB? - matlab

Question

I have the following image derived from imagesc(some matrix whose entries correspond to those colors). The Cyan and the Yellow both mean different things. I would like to either:
Add a legend where I can fill in what each color means
Segregate parts of the X-axis to where I can type "cyan" on the x region below the cyan part, and "yellow" on the x region below the yellow part.
Either or would be fine, and which ever one is easier would be appropriate for me.
CYAN YELLOW

Answer 1

Do you want something like this? It's very basic haha.
clc
clear
close all
%// Dummy array
A = repmat([0 0 0 1 1 1],6,1);
imagesc(A)
hold on
%// Dummy data to add legend
scatter(0,0,1,'b','filled')
scatter(0,0,1,'r','filled')
axis off
colorbar
%// Get axis coordinates (x first and then y)
ax = axis;
%// Add text. You can easily adjust the x-offset depending on how many colors you have.
text(ax(2)/4+ax(1),ax(4)+.2,'Blue','Color','b','FontSize',20,'HorizontalAlignment','Center')
text(3*ax(2)/4+.2,ax(4)+.2,'Red','Color','r','FontSize',20,'HorizontalAlignment','Center')
%// Add legend
legend({'Blue';'Red'})
Output:

Answer 2

Here's another option, which happens to be matlab-hg2 friendly:
%% // Initialization
clear variables; close all force; clc;
%% // Generate some data
fakeData = magic(3)-0.5;
fakeData_horz = fakeData(:)'; %//'
fakeNames = cellstr(strcat('color',num2str((1:9)'))); %//'
fakeNameMapping = fakeNames(randperm(numel(fakeData)));
%% // Create figure
hFig = figure('Position',[680,488,758,610],'Resize','off');
%% // Top left example
cLims = [0 numel(fakeData)+1];
hSp = subplot(2,2,1);
imagesc(fakeData); axis image; set(hSp,'XTick',[],'YTick',[]);
colorbar; caxis(cLims);
[XX,YY] = meshgrid(1:size(fakeData,1),1:size(fakeData,2));
text(XX(:),YY(:),fakeNameMapping,'HorizontalAlignment','center');
%% // Bottom example
hSp = subplot(2,2,3:4);
cLims = [0 numel(fakeData)+1]; %Not required here since unchanged
imagesc(fakeData_horz); axis image; set(hSp,'XTick',[],'YTick',[]);
colorbar; caxis(cLims);
drawnow; %// This command will allow the annotations to be positioned properly
for ind1=1:numel(fakeData_horz)
newPos = [hSp.Position(1)+hSp.Position(3)/numel(fakeData_horz) * (ind1-1),...
hSp.Position(2)*1.6,... %1.6 is chosen for the demo
hSp.Position(3)/numel(fakeData_horz),...
0.05]; % 0.05 is chosen for the demo; play around with it
h= annotation('textbox',newPos,'String',fakeNameMapping{ind1},...
'LineStyle','none','HorizontalAlignment','center');
end
%% // Top right example
hSp = subplot(2,2,2);
cLims = [0 numel(fakeData)]; %// cLims is a bit different here!
imagesc(fakeData); axis image; set(hSp,'XTick',[],'YTick',[]);
caxis(hSp,cLims); colormap(hSp,parula(numel(fakeData)));
cb = colorbar; %// This time we need a handle to the colorbar
cb.Ticks = (hSp.CLim(1):hSp.CLim(2))+0.5; %// Set the tick positions
cb.TickLabels = fakeNames; %// Set the tick strings
Which results in:
Note: unless using a more intelligent text positioning computation, the figure's size should not be changed after it was plotted (in the 2nd example), because then the text no longer remains where it should be.
Edit: added another option where only the colorbar is labeled.