How to Create a Static Legend for a Dynamic Axes

I have written a GUI for interaction with and tracking of the Lorenz Attractor. The 3D plot is in the main figure, but the user can open a separate figure that displays the position-time graph for each coordinate direction. This component plot deletes the data from the graph handle as soon as it leaves the lower axis range to avoid consuming my memory (the data essentially scrolls across the axes). It currently has a very good running speed, but I cannot figure out how to add a legend to the component plot without slowing down performance. Even if I turn auto-update off and only create the legend for the first set of points plotted, it somehow still finds a way to slow down my program significantly as it continues to run. Is there a way to assign the legend initially and then make it entirely static, so it is essentially just an image overlaid on the axis?

Here is my code for the plotting loop that currently has the legend statement commented out

if true
  %animation plotting loop
      ch = 0;
      ck = 0;
      for i = 2:length(t)

            if ~triggers.stop
                triggers.i = i;
                triggers.startPos.Update = [a(i, 1), a(i, 2), a(i, 3)];
                h(i-ch) = plot3(ax1, a(i-1:i,1),a(i-1:i,2),a(i-1:i,3), linestyle, 'color', color);

                if i > nTrim && nTrim ~= 0
                    delete(h(2))
                    h(2) = [];
                    ch = ch+1;
                end
                if triggers.comp && mod(i, 2) == 0 && i > kBuffer
                    xlim(ax3, [t(i)-tGRange, t(i)]);
                    ylim(ax3, [min(min(a)), max(max(a))]);
                    k(:, triggers.kk-ck) = plot(ax3, t(i-kBuffer:i), a(i-kBuffer:i, 1), '-k', t(i-kBuffer:i), a(i-kBuffer:i, 2), '-r', t(i-kBuffer:i), a(i-kBuffer:i, 3), '-c');
%                     if triggers.kk == 2
%                         legend(ax3, k(:,2), {'X Position', 'Y Position', 'Z Position'}, 'AutoUpdate','off')
%                     end
                    if k(1, 1).XData < t(i)-tGRange
                        delete(k(:, 1))
                        k(:, 1) = [];
                        ck = ck+1;
                    end
                    triggers.kk = triggers.kk + 1;

                end

                pause(pauseTime)
            else %break statement for terminating program
                break
            end
        end
        triggers.stop = false;
    end
  end

Answer 1

[Don Zheng]

Try setting the XData/YData/ZData properties of the plots rather than calling 'plot'/'plot3' each time in the loop. For example:

close all;
figure;
ax = axes;
px = plot(ax, 0, 0, 'x'); hold on;
py = plot(ax, 0, 0, 'o');
legend(ax, {'x dir', 'y dir'});

deg = 0:720;
xv = sind(deg);
yv = cosd(deg);
scope_width = 50;
for i = deg+1
    range = i:min(numel(deg),i+50);
    px.XData = deg(range);
    px.YData = xv(range);
    py.XData = deg(range);
    py.YData = yv(range);
    xlim([px.XData(1), px.XData(1)+scope_width]);
    ylim([-1, 1]);
    pause(0.01);
end

[Caleb Thomas]

Thank you! That is much better than the method I had been using, especially when it came to trimming the data. I had not considered simply augmenting the handles data matricies themselves, that is quite crafty. The performance still takes a bit of a hit when the components are opened, naturally, but the effect does not compound over time and overall it still runs at a reasonable speed, but still not at the same speed as no legend at all. I'm afraid it may just be the extra graphics and handle putitng a strain on my system (even though they should be very lightweight?)

Thank you again for your help!

Answer 2

[Breno Vincenzo de Almeida]

In addition to Don Zheng's answer, you can set the legend to auto update off.

Using his example as reference, just do:

lgd = legend(ax, {'x dir', 'y dir'});
lgd.AutoUpdate = 'off';

Source: Legend properties page.