Just got a build on Linux ready. AFAICS stretching of the legend when resizing plot windows is no more - good!

By the way: an impressive piece of work, Panxto, thank you very much!

@Rik: Thanks. I fixed the latest reported issues, improved the autoupdate code, and pushed the patch here:

http://hg.savannah.gnu.org/hgweb/octave/rev/c66467f74278

I posted a new bug in which we can handle the updates to the documentation

https://savannah.gnu.org/bugs/?57254

I also marked the following legend bugs bugs ready for test:

bug #50038, bug #40333, bug #39697, bug #56787, bug #48859, bug #42940, bug #33112, bug #50094, bug #41142, bug #56972, bug #55379.

I attached an updated cset that improves performances a bit and fixes a bunch problems.

(file #47857)

Looks like the zip archive is too large.

(file #47844, file #47845, file #47846, file #47847)

I attached a figure that shows the performance drop when plotting and printing legend demos. In lies between 20% (most often) and more than 100% in some cases.
I also attached a zipped archive in which you will find the printed demos. Open compare_plot_demos_04.html to see the comparison.

I'm starting to recall that legend is a very intricate function with lots of small behavior deviations.  The good news is that the second patch is much faster, and about the same timing as what we have today when used on ordinary plots.

Some additional corner cases that I discovered.

Corner case #1 (incorrectly handling multiple text strings)

x = linspace (0, 2*pi, 100);
plot (x, sin (x), 'o-', x, cos (x), '*-')
legend ("sine", "cosine");
error: binary operator '/' not implemented for 'cell' by 'scalar' operations
error: called from
    legend>__legend_textitem_array__ at line 841 column 14
    legend>__legend_update_layout__ at line 632 column 31
    legend at line 201 column 5
    /tmp/oct-tKWrbi at line 3 column 6

Corner case #2 (labeling more than 20 data objects)

The behavior quoted below is from the existing legend implementation.  Like Matlab, if the default number of labels is greater than 20 then it issues a warning and only labels the first 20.  This behavior is no longer present in the patch.

octave:2> plot (rand (30));
octave:3> legend ()
warning: legend: labeling only first 20 data objects
warning: called from
    legend at line 450 column 11

Corner case #3 (doesn't accept a numeric value for "location")

This could be behavior that we just drop.  Specifying location with a numeric value has been deprecated for a very long time, and is no longer supported in Matlab either.

octave:1> plot (1:10, 'o-')
octave:2> legend ('abc', 'location', 3)
error: set: invalid value for radio property "location"
error: called from
    legend at line 195 column 5

Corner case #3 (legend object created even when function fails)

This is a problem because it creates an axes object with tag "legend" which leaves a white rectangle on the plot.  If input validation fails there should be no object created.  Maybe you want to surround the construction of the axes object with a try/catch block, and if there is an error delete the created axes legend object and then re-throw the error.

octave:5> plot (1:10, 'o-')
octave:6> h = legend ('abc', 'location', 'foobar')
error: invalid value = foobar
error: called from
    legend at line 195 column 5
octave:7> findall (gcf, 'tag', 'legend')
ans = -5.6867

Corner case #4 ("bestoutside" location property not implemented)

Current legend.m implements "bestoutside" so we shouldn't regress on that behavior.

octave:10> plot (1:10, 'o-')
octave:11> h = legend ('abc', 'location', 'bestoutside')
error: invalid value = bestoutside
error: called from
    legend at line 195 column 5

Corner case #5 ("none" location property not implemented)

"none" turns off the default positioning algorithm and allows the user to set the location directly with the "position" property of the legend object.

octave:10> plot (1:10, 'o-')
octave:11> h = legend ('abc', 'location', 'none')
error: invalid value = none
error: called from
    legend at line 195 column 5

Comment #6 (Octave prefers no Camel Case properties)

The following properties should probably be all lower case to match Octave's other properties.

 AutoUpdate = off
 NumColumns = 1
 NumColumnsMode = auto

Corner case #7 (listener on "numcolumns"?)

The "numcolumns" property works correctly when the legend is created, but subsequent changes to the property don't re-format the legend.

x = linspace(0,pi);
y1 = cos(x);
plot(x,y1)

hold on
y2 = cos(2*x);
plot(x,y2)

y3 = cos(3*x);
plot(x,y3)

y4 = cos(4*x);
plot(x,y4)
hold off

hl = legend({'cos(x)','cos(2x)','cos(3x)','cos(4x)'},'Location','northwest','NumColumns',2);

set (hl, 'numcolumns', 1)

Corner case #8 (legend show)

When called with the argument "show", legend should display a legend as if it was called with no arguments.  This produces "data1", "data2", etc.  Instead, the new legend code is treating "show" as a text label rather than a command.  legend ("toggle") produces the same behavior.

octave:106> plot (rand (3,2))
octave:107> legend show
warning: legend: ignoring extra objects.
warning: called from
    legend>__legend_parse_opts__ at line 555 column 9
    legend at line 125 column 8

Corner case #9 (horizontal spacing)

Try

demo legend 6

and you will see that the text and keys are crowded when compared to the current version of legend.

Corner case #10 (property "textcolor" not implemented?)

Try

demo legend 9

The text should be magenta color rather than black.

Corner case #11 (demo #15, #16 produce warnings when they shouldn't)

legend example 15:
 clf;
 x = linspace (0, 10);
 plot (x, x);
 hold on;
 stem (x, x.^2, "g");
 title ("First created object gets first label");
 legend ("linear");
 hold off;

warning: legend: ignoring extra objects.
warning: called from
    legend>__legend_parse_opts__ at line 555 column 9
    legend at line 125 column 8
    __demo__ at line 9 column 2
    demo at line 147 column 7

Corner case #12 (demo #22 produces only one label)

Try

demo legend 22

It should produce a legend with "sin (x)" and "cos (x)" as labels.

Corner case #13 (callback execution error when running demo #28)

This occurred while running all of the demos in legend.m

Press <enter> to continue:
execution error in graphics callback function
legend example 28:
 clf;
 x = 0:10;
 y1 = rand (size (x));
 y2 = rand (size (x));
 [ax, h1, h2] = plotyy (x, y1, x, y2);
 ylabel (ax(1), {"Blue", "Y", "Axis"});
 title ('plotyy legend test #2: "westoutside" adjusts to ylabel');
 drawnow ();
 legend ([h1, h2], {"Blue", "Orange"}, "location", "westoutside");

Corner case #14 (demo #29 fails)

Try

legend demo 29

The rightside axes is messed up.

Corner case #15 (demo #30)

In the new legend code, the subplots are all set to the same size which accommodates the largest of the text strings.  In previous versions of Octave, in Matlab, each subplot is re-sized to fit the length of the label it is paired with.

Try

legend demo 30

Comment #16 (demo #36 missing)

Demo #36 is missing in the new code.  I think it is correct to include it because it shows that the problem has now been fixed.  If there is a regression in behavior this demo will show it.

Comment #17 (performance measure)

I used demo #14 which does not include subplots to show that the new code can be significantly slower even in an ordinary case.

tic; demo ("legend", 14); toc

Old code: 0.5 seconds
New code: 2.3 seconds

Comment #18 (length of lines in legend key)

I used

demo legend 3

and then measured the length of the lines in the legend key.  In the old code it is about 20 pixels and in the new code it is about 27 pixels.  I like the shorter length.  Matlab uses long line lengths and I think it is ugly.

Comment #19 (hide _appdata_ property?)

The _appdata_ property is for internal use only.  It should probably be a hidden property so that users don't have to see all the guts of the implementation.

octave:149> plot (1:10, 'o-')
octave:150> hl = legend ('abc')
hl = -102.66
octave:151> get (hl)
ans =

  scalar structure containing the fields:

    __appdata__ =

      scalar structure containing the fields:

        __axes_handle__ = -108.94
        __next_label_index__ = 1
        __peer_objects__ = -103.14
        __updating_layout__ = 0
        __original_looseinset__ =

           0.130000   0.110000   0.095000   0.075000

        __original_units__ = normalized
        __peer_axes_position__ =

            54.587    34.641   325.420   256.662

        __listeners__ =
        {
          [1,1] =
          {
            [1,1] = 1
            [1,2] = position
            [1,3] =
            {
              [1,1] = @__maybe_update_layout__
              [1,2] = -102.66
            }

          }

          [1,2] =
          {
            [1,1] = -108.94
            [1,2] = position
            [1,3] =
            {
              [1,1] = @__maybe_update_layout__
              [1,2] = -102.66
            }

          }

          [1,3] =
          {
            [1,1] = -108.94
            [1,2] = tightinset
            [1,3] =
            {
              [1,1] = @__maybe_update_layout__
              [1,2] = -102.66
            }

          }

          [1,4] =
          {
            [1,1] = -108.94
            [1,2] = colormap
            [1,3] =

@(hax) set (hl, "colormap", get (hax, "colormap"))

          }

          [1,5] =
          {
            [1,1] = -108.94
            [1,2] = fontsize
            [1,3] =

@(hax) set (hl, "fontsize", 0.9 * get (hax, "fontsize"))

          }

          [1,6] =
          {
            [1,1] = -108.94
            [1,2] = children
            [1,3] =
            {
              [1,1] = @__legend_autoupdate__
              [1,2] = -102.66
            }

          }

        }


    AutoUpdate = off
    edgecolor =

       0.1500   0.1500   0.1500

    interpreter = tex
    itemhitfcn = [](0x0)
    location = northeast
    NumColumns = 1
    NumColumnsMode = auto
    orientation = vertical
    string =
    {
      [1,1] = abc
    }

Corner case #20 ("EdgeColor" not implemented?)

Using this property doesn't seem to work anymore.

plot (1:10, 'o-');
hl = legend ('abc');
set (hl, 'edgecolor', 'r')

If this gets checked in you should be able to close a large number of bug reports that all relate to legend not adjusting in size correctly with changes in the axes and figure.  I just came across another one: bug #56972.

I forgot to mention the ugly hack ... since I know nothing about gnuplot, I renamed the old legend.m code to _gnuplot_legend_.m and call this function when gnuplot is the graphics toolkit.

I eliminated the questionable use of the "userdata" property to hold internally private legend data in this cset (http://hg.savannah.gnu.org/hgweb/octave/rev/19d63669d174).

Not that it is relevant for this task, but I significantly rewrote legend.m for greater Matlab compatibility in this cset (http://hg.savannah.gnu.org/hgweb/octave/rev/436deae9f9ab).

Here are a few other tests inspired by the current demos

%% Test the current demos work
%% demo 1: currently fails because text object are deleted/recreated
hf = figure ('visible', 'off');
plot (rand (2));
[hl, hicon] = legend ({"foo"}, "bar");
legend (hl, "location", "northeastoutside");
set (hl, "fontsize", 20);
ht = hicon(strcmp (get (hicon, "type"), "text"));
assert (get (hl, "string"), {"foo", "bar"});
assert (get (ht, "string"), {"foo"; "bar"});
assert (get (ht, "fontsize"), [20; 20])
assert (get (hl, "location"), "northeastoutside")
close (hf);

%% demo 2:
hf = figure ('visible', 'off');
plot (rand (3));
hl = legend ("show");
assert (get (hl, "string"), {"data1", "data2", "data3"})
close (hf);

%% demo 3: fails because legend without arguments doesn't return hicon
hf = figure ('visible', 'off');
x = 0:1;
hlines = plot (x,x,";I am Blue;", x,2*x, x,3*x,";I am yellow;");
[hl, hicon] = legend ();
hleglines = hicon(strcmp (get (hicon, "type"), "line"));
assert (get (hlines(3:-2:1), "color"), get (hleglines, "color"));
close (hf);

%% demo 4:
hf = figure ('visible', 'off');
plot (1:10, 1:10, 1:10, fliplr (1:10));
hl = legend ({"I am blue", "I am orange"}, "location", "east");
legend ("hide")
assert (get (hl, "visible"), "off")
hl2 = legend ("show");
assert (get (hl2, "visible"), "on")
assert (hl, hl2)
close (hf);

%% demo 5:
hf = figure ('visible', 'off');
plot (1:10, 1:10, 1:10, fliplr (1:10));
[hl, hicon] = legend ({"I am blue", "I am orange"}, ...
                      "location", "east", "orientation", "horizontal");
ht = hicon(strcmp (get (hicon, "type"), "text"));
assert (get (ht(1), "position")(1) < get (ht(2), "position")(1))
legend boxoff
assert (get (hl, "box"), "off")
legend boxon
assert (get (hl, "box"), "on")
close (hf);

%% demo 7:
hf = figure ('visible', 'off');
plot (1:10, 1:10, 1:10, fliplr (1:10));
legend ({"I am blue", "I am orange"}, "location", "east");
[hl, hicon] = legend ("left");
ht = hicon(strcmp (get (hicon, "type"), "text"));
hlines = hicon(strcmp (get (hicon, "type"), "line"));
assert (get (ht(1), "position")(1) < get (hlines(1), "xdata")(1))
assert (get (ht(2), "position")(2) < get (hlines(2), "xdata")(2))
close (hf);

%% demo 8
hf = figure ('visible', 'off');
plot (1:10, 1:10, 1:10, fliplr (1:10));
[hl hicon] = legend ({"I am blue", "I am orange"}, "location", "east");
ht = hicon(strcmp (get (hicon, "type"), "text"));
set (hl, "textcolor", [1 0 1]);
assert (get (ht, "color"), {[1 0 1]; [1 0 1]});
close (hf);

%% demo 14
hf = figure ('visible', 'off');
hlines = plot (rand (2));
[hl hicon] = legend ("foo");
ht = hicon(strcmp (get (hicon, "type"), "text"));
hlegline = hicon(strcmp (get (hicon, "type"), "line"));
assert (get (ht, "string"), get (hlines(1), "displayname"))
assert (get (hlegline, "color"), get (hlines(1), "color"))
close (hf);

%% demo 17
hf = figure ('visible', 'off');
subplot (2,1,1);
rand_2x3_data1 = [0.341447, 0.171220, 0.284370; 0.039773, 0.731725, 0.779382];
hbars = bar (rand_2x3_data1);
[hl hicon] = legend ({"1st Bar", "2nd Bar", "3rd Bar"}, ...
                     "location", "northwest");
hicon = hicon(! strcmp (get (hicon, "type"), "text"));
assert (get (hicon, "type"), {"patch"; "patch"; "patch"})
assert (get (hicon, "facecolor"), get (hbars, "facecolor"))
subplot (2,1,2);
x = linspace (0, 10, 20);
hstem(1) = stem (x, 0.5+x.*rand (size (x))/max (x), ...
                 "markeredgecolor", [0 0.7 0]);
hold on;
hstem(2) = stem (x+10/(2*20), x.*(1.0+rand (size (x)))/max (x));
[hl hicon] = legend ({"Multicolor", "Unicolor"}, "location", "northwest");
hicon = hicon(! strcmp (get (hicon, "type"), "text"));
assert (get (hicon, "type"), {"line"; "line"})
assert (get (hicon, "color"), get (hstem, "markeredgecolor"))
close (hf);

%% demo 18
hf = figure ('visible', 'off');
surf (peaks ());
[hl hicon] = legend ("peaks()");
hicon = hicon(! strcmp (get (hicon, "type"), "text"));
assert (get (hicon, "type"), "patch")
close (hf);

%% demo 21/22/23
hf = figure ('visible', 'off');
x = 0:0.1:10;
plot (x, sin (x), ";sin (x);");
hold on;
plot (x, cos (x), ";cos (x);");
[hl hicon] = legend ();
assert (get (hl, "string"), {"sin (x)", "cos (x)"});
hl = legend ({"Sine", "Cosine"}, "location", "northeastoutside");
assert (get (hl, "string"), {"Sine", "Cosine"});
close (hf);

%% demo 24
hf = figure ('visible', 'off');
plot (1:10);
hl = legend ("foo");
legend off
assert (! ishandle (hl))
close (hf);

%% demo 25
hf = figure ('visible', 'off');
plot (rand (2));
legend ("Hello", "World", "interpreter", "foobar");
assert (lastwarn (), "legend: ignoring extra labels")
close (hf);

%% demo 26/27/28
hf = figure ('visible', 'off');
x = 0:10;
y1 = rand (size (x));
y2 = rand (size (x));
[hax, h1, h2] = plotyy (x, y1, x, y2);
hl = legend ("Blue", "Orange");
assert (get (hl, "string"), {"Blue", "Orange"})
hl = legend ({"Blue", "Orange"});
assert (get (hl, "string"), {"Blue", "Orange"})
hl = legend ([h1, h2], {"Blue", "Orange"});
assert (get (hl, "string"), {"Blue", "Orange"})
close (hf);

Can bug #49446 added to the list, please? (at least for legends)