How do I prevent the legend from REALLY auto-updating

I'm plotting graphs of the same type in multiple subplots, with some subplots left blank. I'd like to use the blank spots to plot a legend. I could of course move the location of the legend but that's really tedious. The seemingly obvious, clean way to do this is as in the following code. In the sixth plot region, I plot my lines, create the legend, set autoupdate to off, then set the linestyles to 'none'. Trouble is: when I set the linestyles to none, the legend autoupdates and sets the legend lines to none as well. So what I need to do is break the link between the actual lines and the legend lines.

close all;
for ii=1:2:9;
    subplot(3,3,ii);
    clear h;
    hold on;
    h(1) = plot(1:10);
    h(2) = plot(10:-1:1);
    if ii==5;
        subplot(3,3,6);
        hold on;
        h(1) = plot(1:10);
        h(2) = plot(10:-1:1);
        legend('up','down','AutoUpdate','off');
        for ii=1:numel(h);
            h(ii).LineStyle='none';
        end;
    end;
end;

A hideous kludge is the following: close all;

for ii=1:2:9;
    subplot(3,3,ii);
    clear h;
    hold on;
    h(1) = plot(1:10);
    h(2) = plot(10:-1:1);
    if ii==5;
        subplot(3,3,6);
        hold on;
        h(1) = plot(11:20);
        h(2) = plot(20:30);
        legend('up','down','AutoUpdate','off');
        set(gca,'YLim',[1,10]);
    end;
end

But this is unsatisfactory in so many ways that I won't bother to enumerate them. So the question is, is there a clean way to do what I want to do, just by breaking the link between actual plot lines and legend lines, and not resorting to a hideous kludge?

[dpb]

The 'autoupdate' only determines whether the legend elements are added/deleted when objects are added/deleted from the axes; not whether the existing entries will/will not be kept in synch with their objects (as you've determined).

Only way I see to do something of the sort would be to use a pair of line objects specifically and only for the purpose of displaying their legend; use the form of legend for the specific handles only instead of generic for whichever are the first N lines matching the number of label strings passed.

[Leo Simon]

I'm afraid I don't follow the suggestion. @dpb, would you mind modifying my first example to illustrate please?

Answer 1

[dpb]

Actually, it's simpler than outlined; that kept extra lines around; don't even need that; just use the builtin feature of plot() to not show NaN values...

for ii=1:2:nSubplots
  ...
  subplot(3,3,ii);
      ...
    plots here
end
hAx=subplot(3,3,6);    % for legend
hL(1)=plot(nan(1,1));  % create the line style; doesn't show
hL(2)=plot(nan(1,1));
hLg=legend('up','down','AutoUpdate','off');
delete(hL);            % can even remove the objects
hAx.Color='none';      % just leave the legend visible
hAx.XColor='none';
hAx.YColor='none';

Salt to suit re: box outline, etc., etc., etc., ...

ADDENDUM Better way to hide the unwanted axis than zzz.Color='none' is

hAx.Visible='off';

[Leo Simon]

Perfect, thanks very much indeed

Answer 2

[Jan]

Creating the axes object and lines only to hide them later sounds indirect and cluttered. What about setting the 'Position' of the legend directly?

figure;
for ii=1:2:9;
  subplot(3,3,ii);
  hold on;
  h(1) = plot(1:10);
  h(2) = plot(10:-1:1);
end

% Use last set of handles:
LegH = legend(h, {'up', 'down'}, 'Position', [0.7548, 0.5123, 0.1250, 0.0821]);

Of course, now the problem is how to find the needed value for the position automatically. I'd move this to an extra function, which finds the position of the subplot and uses it for moving the legend. This function will not look very smart also, but at least you do not have to create and hide graphics objects and struggle with auto-updates.

[Leo Simon]

Thanks Jan, I really don't want to have to move the legend Position (I did this in the past) for precisely the reason you indicated, but it may be a choice between bad options.