legend showing wrong colours

Hi,

I want to plot 4 different arrays and their corresponding trendlines. In order to discriminate between those 4 I want them to have their own colour. Unfortunately my legend doesn´t correspond to the plots (see attached screenshot) - it should only show "Graph A1", "Graph A2", "Graph A3" and Graph A4" with four different colours.

j = 1;
k = 1;
str = {'A1', 'A2', 'A3', 'A4'};
col = {'r', 'k', 'g', 'b'};
for i = 1:20:80
    plot(M_3(i:19+i), col{k}, 'LineWidth',1); 
    str = [str ("Graph " + str(j))]; 
    c_rms = polyfit(time,M_3(i:19+i),1);
    y_est = polyval(c_rms,time);
    hold on
    plot(time,y_est, col{k}, 'LineWidth',2);
    j = j+1; 
    k = k+1; 
end
set(gcf,'position',[0 500 1000 300])
xlabel('time / min');
ylabel('electrical activity in % of the maximal force');
grid on; grid minor;
legend(str);

I would be very happy if you could help me out, maybe I just missordered the loop.

Thanks,

Robin

Answer 1

[Adam Danz]

The easiest way to manage legend text is by using the DisplayName property of graphics objects. Here's how that might look when plotting in a loop.

str = {'A1', 'A2', 'A3', 'A4'};
hold on
for i = 1:4
    % . . . skipping stuff
    plot(x,y,'DisplayName', str{i})
    plot(xFit,yFit,'DisplayName', [str{i},' fit'])
end
legend()

If you only want some objects to appear in the legend,

str = {'A1', 'A2', 'A3', 'A4'};
hold on
handles = gobjects(4,1); 
for i = 1:4
    % . . . skipping stuff
    handles(i) = plot(x,y,'DisplayName', str{i}); 
    plot(xFit,yFit)
end
legend(handles)

[Robin Strak]

Thanks for you quick answer, Adam!

I tried to incorporate your tip, but in my case there still a little bug:

As I want to plot M_3 in 4 parts I looped it with for j = 1:20:80, so in order to choose the displayed string (A1,A2, ...) I need another loop - as you descriped.

time = linspace(0,22,20);

str = {'A1', 'A2', 'A3', 'A4'};

hold on

for i = 1:4

for j = 1:20:80

plot(time, M_3(i:19+i),'DisplayName', str{i})

j = j+1;

end

end

legend()

I´ve tried that but now I´ve got 16 graphs plotted. Do you think I could solve it with only one loop?

Thanks,

Robin

[Adam Danz]

Instead of setting up your loop like this

for i = 1:20:80
    plot(M_3(i:19+i), . . .); 
    . . .
end

set it up like this

vec = 1:20:80; 
for i = 1:numel(vec)
    plot(M_3(vec(i):19+vec(i)), . . .); 
    %        ^^^^^^    ^^^^^^
    . . .
end

That way your loop uses integer values 1:n that can be used as indices.

Here's a similar example.