Given that gnuplot is no longer supported by Octave and this report is 6 years old, I am closing this report.

I expanded the code to better see how scatter is getting a different color in gnuplot than in fltk.

close all
x = rand (1,1);
y = rand (1,1);
colors = rand (1,1);

graphics_toolkit gnuplot
scatter (x,y, [], colors, "filled")
colormap (rainbow (3));

title ("gnuplot");
graphics_toolkit fltk
figure
scatter (x,y, [], colors, "filled")
colormap (rainbow (3));
title ("fltk");

With only three colors in the colormap, gnuplot always chooses the last one while fltk always chooses the middle one.

Since it was uncertain whether this will be the final bug fix for the error legend I pushed the cset to default (http://hg.savannah.gnu.org/hgweb/octave/rev/54332b6ddeb6).

OK, I think I know what is going on.  The error bars were removed:

Changeset:
10597 (ba346313bdc1) __go_draw_axes__.m: Remove gnuplot errorbar code.

I'm guessing because Octave plotting routines should take control of drawing the error bars rather than have gnuplot do so.  I think what we should do for the key in this case is not add any entries for the lines, but instead draw some phantom lines having maybe only a NAN entry and be of the proper error bar style.  Otherwise, there is no way to get a proper error bar key entry.

For the problems with the '@' character, if the "interpreter" property is set to "none" then they disappear.  In the code, Octave replaces '@' with '\@', but this regexprep no longer works.  Apparently, the replacement pattern now has to be '\\@'

if (enhanced)
  str = regexprep (str, '(?<!\\)@', '\@');
endif

I made that change here http://hg.savannah.gnu.org/hgweb/octave/rev/2bca08793651.

I ran dump_plot_demos today (10/31/16) to determine the state of the gnuplot toolkit.  I don't think any of these are critical to the 4.2 release, but it would be good to address them, particularly the regressions, at some point.

FUNCTION    DEMO #      ISSUE
errorbar    3,4         Four keys in legend rather than two

ezsurfc     1           Title too low

area        2           Superscript in title is truncated by position onscreen

autumn      1           Lines which appear directly on an axis are not shown.
                        This should be controlled by the axis "layer" property.
                        The default is "bottom" which means user drawn lines
                        should appear above the axes lines.
                        All of the demos which use rgbplot tend to have this problem.

axis        3           "axis off" still shows box around axis

axis        9           X and Y axis lines appear in color, rather than black.
                        This is associated with using a different "origin" than
                        the default.  It is probably using the next linestyle
                        or something like that.

box         4           The property "boxstyle" = "back" is not implemented.

contour     3           The single contour is drawn with the last color in the
                        colormap rather than the intermediate value (~128).

contourf    3           The filled contour is drawn with the last color in the
                        colormap rather than the intermediate value.

fplot       1           The legend tag is transparent, rather than having a
                        solid white background.  I think this has been reported
                        already.

fplot       3           Legend text which uses '@' has that character deleted
                        and the spacing of the following characters is messed
                        up.  There is special code to deal with the '@' char
                        in __gnuplot_draw_axes__.m, but it seems to be failing.
                        This is a regression from 4.0.3.

legend      9           "legend hide" does not hide legend.  This is a
                        regression from 4.0.3.

legend      11          Greek text causes alignment of text to be off.

legend      13          Large number of keys in legend causes overlap of X label

legend      18          Color patch missing from legend key for surface object

legend      26          Legend not contained within axes box

legend      31          Long key text extends outside legend box

mesh        4           Title within axes box

scatter     1           Plotted MarkerSize is smaller than for OpenGl/Matlab

scatter     7           For the case of Random Color: 1 Point, the single point
                        is in yellow (max of colormap) rather than a greenish
                        color.

slice       2           Fill color of mesh outside of central area is yellow,
                        rather than white or possibly transparent.

subplot     1           Titles and xlabels overlap