Add custom legend without any relation to the graph

Question

I wish to insert a legend that is not related to the graph whatsoever:

figure;
hold on;
plot(0,0,'or');
plot(0,0,'ob');
plot(0,0,'ok');
leg = legend('red','blue','black');

Now I wish to add it to another figure:

figure;
t=linspace(0,10,100);
plot(t,sin(t));
%% ADD THE LEGEND OF PLOT ABOVE 

Answer 1

This is how I have solved this problem in the past:

figure
t=linspace(0,10,100);
plot(t,sin(t));
hold on;

h = zeros(3, 1);
h(1) = plot(NaN,NaN,'or');
h(2) = plot(NaN,NaN,'ob');
h(3) = plot(NaN,NaN,'ok');
legend(h, 'red','blue','black');

This will plot the additional points, but because the coordinates are at NaN they will not be visible on the plot itself:

EDIT 26/10/2016: My original answer results in greyed out legend entries in 2016b. The updated code above works, but the answer below is only relevant pre-2016b:

figure
t=linspace(0,10,100);
plot(t,sin(t));
hold on;

h = zeros(3, 1);
h(1) = plot(0,0,'or', 'visible', 'off');
h(2) = plot(0,0,'ob', 'visible', 'off');
h(3) = plot(0,0,'ok', 'visible', 'off');
legend(h, 'red','blue','black');

This will plot the additional points, but they will not be visible on the plot itself.

You can also use copyobj to copy graphics elements from one figure to another if you have a lot of elements, then use set(x, 'visible', 'off') to hide them before showing the legend, but it depends on what your final application is.

Answer 2

Your question is a little unclear. However, the first thing I thought of when reading it was the text function in Matlab.

You can use the text function to add text to a Matlab figure. It's use is

>> text(x, y, str);

where x and y are the coordinates in the figure where you want to add the text str. You can use the Color option of text for colours and TeX to draw lines or even _. I've gotten very creative with plots using text.

Here's a quick and dirty example of emulating a legend with text

x = 0:pi/20:2*pi;
y = sin(x);
plot(x,y)
axis tight

legend('sin(x)');

text(5.7, 0.75, 'sin(x)');
text(5.1, 0.78, '_____', 'Color', 'blue');

which produces

             

For this specific case you could use the specific command (noted by @Hoki in the comments).

ht = text(5, 0.5, {'{\color{red} o } Red', '{\color{blue} o } Blue', '{\color{black} o } Black'}, 'EdgeColor', 'k');

to produce

             

by retrieving the handle to the text object it becomes trivial to copy it to a new figure, copyobj(ht, newfig). [1]