How do I get a "tabular" legend?

In MatlabR2015b, I am plotting a 2D scatter plot of "Performance" that is a function of two independent variables "Active agent" & "Temp". I have 3 Active Agents and 4 Temps. In scatter plot, I am using a 12 colors, one for each (ActiveAgent,Temp) pair. I want to customize legend so that it appears in a tabular form like (C1 would be like marker 'o' in its respective colors)

       Temp1 Temp2 Temp3 Temp4
Agent1   C1    C2    C3    C4
Agent2   C5    C6    C7    C8
Agent3   C9    C10   C11   C12

How can I do this?

Answer 1

[Kelly Kearney]

You can hack your way to a solution with my legendflex function. It's designed to make gridded-style legends like this, though unfortunately doesn't include the row/column label function yet (I'll keep that in mind for an enhancement, though... shouldn't be too hard to add).

Scatter only creates a single object to label, so to get individually-colored marker for a legend, you can create some invisible line objects:

% Data

nrow = 3;
ncol = 4;
ngrp = nrow*ncol;

x = rand(100,1);
y = rand(100,1);
group = ceil(rand(100,1)*ngrp);

datalabel = strtrim(cellstr(num2str((1:ngrp)', 'C%d')));
rowlabel = strtrim(cellstr(num2str((1:nrow)', 'Agent%d')));
collabel = strtrim(cellstr(num2str((1:ncol)', 'Temp%d')));

% Plot

scatter(x,y,[],group);
cmap = parula(ngrp);
colormap(cmap);
set(gca, 'clim', [0 ngrp] + 0.5, 'ylim', [-0.5 1.5]);

hln = line(nan(2,ngrp), nan(2, ngrp), 'marker', 'o', 'linestyle', 'none');
set(hln, {'color'}, num2cell(cmap,2));

% A regular legend to show the hidden-marker idea

hplain = legend(hln(:), datalabel(:));
export_fig('leggrid1', '-png');
delete(hplain);

There are two ways I can think of to get the grid layout. One is to add extra no-marker objects for both the row and column labels. The second is to add extra markers just for the row labels, and create multiple legends with legendflex's title property for the column labels. The latter centers the column titles over the label and object together, which I prefer, but it means you have to keep the box off (which may or may not work for your data):

% Option 1: add extra objects for row/column labels

hr = line(nan(2,nrow), nan(2,nrow), 'linestyle', 'none');
hc = line(nan(2,ncol+1), nan(2,ncol+1), 'linestyle', 'none');

allobj = [hc'; [hr reshape(hln,nrow,ncol)]];
alllbl = ['-' collabel'; [rowlabel reshape(datalabel,nrow,ncol)]];

hleg1 = legendflex(allobj(:), alllbl(:), 'nrow', nrow+1, 'xscale', 0.5);

% Option 2, multiple legends with titles

allobj = [hr reshape(hln,nrow,ncol)];
alllbl = [rowlabel reshape(datalabel,nrow,ncol)];
ttl = ['-' collabel'];

for ii = (ncol+1):-1:1
    if ii == (ncol+1)
        hleg2(ii) = legendflex(allobj(:,end), alllbl(:,end), 'ref', gca, ...
            'anchor', {'se','se'}, 'buffer', [-10 10], 'xscale', 0.5, ...
            'box', 'off', 'title', collabel{end});
    else
        hleg2(ii) = legendflex(allobj(:,ii), alllbl(:,ii), 'ref', hleg2(ii+1), ...
            'anchor', {'sw','se'}, 'buffer', [0 0], 'xscale', 0.5, ...
            'box', 'off', 'title', ttl{ii});
    end
end

[820408]

Thank you for your answer. This is what I was looking for. However, I need to remove "C1" "C2" .... text from the current solution. I added this in my question to indicate the marker color. I should have mentioned that. My mistake. How can I do that?

[Kelly Kearney]

That makes things a little more complicated with legendflex, since it can't handle completely blank labels (hence the little dash I added to the corner). For just straight symbols, I would probably forgo a legend altogether and just manually build a legend-like axis:

% Data

nrow = 3;
ncol = 4;
ngrp = nrow*ncol;

x = rand(100,1);
y = rand(100,1);
group = ceil(rand(100,1)*ngrp);

datalabel = strtrim(cellstr(num2str((1:ngrp)', 'C%d')));
rowlabel = strtrim(cellstr(num2str((1:nrow)', 'Agent%d')));
collabel = strtrim(cellstr(num2str((1:ncol)', 'Temp%d')));

% Plot

scatter(x,y,[],group);
cmap = parula(ngrp);
colormap(cmap);
set(gca, 'clim', [0 ngrp] + 0.5, 'ylim', [-0.5 1.5]);

% "Legend"

[ypt, xpt] = ndgrid(1:nrow, 1:ncol);
pos = get(gca, 'position');
sz = [0.4 0.2];
legax = axes('position', [pos(1)+pos(3)-sz(1) pos(2)+pos(4)-sz(2) sz], ...
    'xlim', [-1 ncol+1], 'ylim', [-1 nrow+1], 'ydir', 'reverse', ...
    'xtick', [], 'ytick', [], 'box', 'on');
hold on;
scatter(xpt(:), ypt(:), [], 1:ngrp);
text(1:ncol, zeros(1,ncol), collabel, 'horiz', 'center');
text(ones(1,nrow)*0.5, 1:nrow, rowlabel, 'horiz', 'right');

[820408]

Thank you. This is what I was looking for.

Answer 2

[Shane L]

If you are looking for a code that is built to create a "tabular" legend, my GridLegend function on the File Exchange will create a "legend" (actually a legend-like axis) with row and column labels. My GridLegend function is not as versatile as Kelly's legendflex function, but it will give you a "tabular" legend with no additional work on the user's end.

Answer 3

[Steven Lord]

In release R2018a, you can now control how many columns your legend has.

% Generate data and set up the axes
x = linspace(0, 2*pi, 500);
axis([0 2*pi -8 8]);
hold on

% Plot 8 sine curves, naming each n*sin(n*x) for legend purposes
for n = 1:8
  plot(x, n.*sin(n*x), 'DisplayName', sprintf('%d*sin(%d*x)', n, n));
end

% Turn on the legend
L = legend('show');

% Make it 2-by-4 instead of 8-by-1
L.NumColumns = 4;

This isn't exactly what the original poster asked for, but it's close.