#5372 [gnuplot_svg_background,false] writes set obj 1 fc rgb NIL

[David Scherfgen]

This bug was discovered by Claude Opus 5.5 and reproduced on Maxima Git commit c62924c (2026-09-26) with SBCL 2.2.9.

Where: src/gnuplot_def.lisp, PLOT-PREAMBLE (gnuplot and xmaxima methods)

The gnuplot and xmaxima PLOT-PREAMBLE methods always print the background color, and RGB-COLOR of false is NIL. So a false background, which the documentation describes as leaving the default of the graphic program, produces invalid commands.

Steps

1. Verify that the existing behavior is incorrect: run the reproducer below; it gives the result under "Before the fix". Judge for yourself that this is wrong instead of relying on the ticket: where the bug allows it, try some related inputs of your own, or check the result another way, for example numerically or against the documentation.
2. Edit the files src/gnuplot_def.lisp, src/xmaxima_def.lisp as described under "Fix".
3. Rebuild Maxima.
4. Verify that the behavior is now correct: the reproducer gives the result under "Correct result". Judge for yourself that this is right, not only that it matches: where the bug allows it, try some related inputs of your own, including some that the fix should leave unchanged.
5. Add the test case(s) under "Test cases" to tests/rtest_plotoptions.mac, at the end of the file.
6. Run the complete test suite, share tests included (make check, or run_testsuite(share_tests=true); in ./maxima-local), and check that the change breaks no test. Failures that also occur without the change (a run on the unchanged build shows which) are pre-existing and can be ignored.
7. Add the line * \#5372 [gnuplot_svg_background,false] writes set obj 1 fc rgb NIL to ChangeLog, under "Bug fixes for numbered bugs:".
8. Commit the fix; a proposed Git commit message is under "Commit message".
9. Push the commit. Then set this ticket's status to "closed" and add a comment naming the commit, for example Fixed by commit [1a2b3c]., with the first six characters of the commit's SHA in the brackets (git rev-parse HEAD | cut -c1-6 prints them). SourceForge links this to the commit.

Reproducer

plot2d(sin(x),[x,0,1],[gnuplot_svg_background,false],[plot_format,gnuplot],[run_viewer,false],[gnuplot_script_file,"/tmp/b28.gp"])$ printfile("/tmp/b28.gp")$

Before the fix:

set obj 1 fc rgb NIL fs solid 1.0 noborder (xmaxima format: {background NIL})

Correct result:

no background object and no background entry

Fix

In src/gnuplot_def.lisp (PLOT-PREAMBLE for GNUPLOT-PLOT, after ";; background color"), replace

        (format dest "set obj 1 rectangle behind from screen 0.0,0.0 to screen 1.0,1.0~%")
        ;; options specific to plot3d
        (format dest "set obj 1 fc rgb ~s fs solid 1.0 noborder~%"
                (rgb-color (getf plot-options '$background_color)))

with

        (when (getf plot-options '$background_color)
          (format dest "set obj 1 rectangle behind from screen 0.0,0.0 to screen 1.0,1.0~%")
          (format dest "set obj 1 fc rgb ~s fs solid 1.0 noborder~%"
                  (rgb-color (getf plot-options '$background_color))))
        ;; options specific to plot3d

In src/xmaxima_def.lisp (PLOT-PREAMBLE for XMAXIMA-PLOT, line "(format st \"-background ~a -data {~%\""), replace

               (format st "-background ~a -data {~%"

with

               (format st "~@[-background ~a ~]-data {~%"

In src/xmaxima_def.lisp (PLOT-PREAMBLE for XMAXIMA-PLOT, line "(t (format st \"{~a {background ~a} \""), replace

              (t (format st "{~a {background ~a} "

with

              (t (format st "{~a ~@[{background ~a} ~]"

Test cases

Add these lines to tests/rtest_plotoptions.mac, separated from the tests before them by two empty lines:

/* Bug #5372: "[gnuplot_svg_background,false] writes set obj 1 fc rgb NIL" */

block ([gpfile: sconcat ("rtest_plotoptions_", ?getpid (), ".gnuplot"),
        script_lines: lambda ([file, pl],
          block ([s: openr (file), l, r: []],
            while stringp (l: readline (s)) do
              if some (lambda ([p], ssearch (p, l) = 1), pl)
                then r: endcons (l, r),
            close (s),
            r)),
        r],
  reset_plot_options (),
  r: [script_lines (
        first (plot2d (sin(x), [x, 0, 1], [gnuplot_svg_background, false],
                       [plot_format, gnuplot], [run_viewer, false],
                       [gnuplot_script_file, gpfile])),
        ["set obj 1"]),
      script_lines (
        first (plot2d (sin(x), [x, 0, 1], [gnuplot_svg_background, red],
                       [plot_format, gnuplot], [run_viewer, false],
                       [gnuplot_script_file, gpfile])),
        ["set obj 1"])],
  reset_plot_options (),
  r);
[[], ["set obj 1 rectangle behind from screen 0.0,0.0 to screen 1.0,1.0",
      "set obj 1 fc rgb \"#ff0000\" fs solid 1.0 noborder"]]$

block ([s1: make_string_output_stream (), s2: make_string_output_stream (),
        show_openplot: false, r],
  reset_plot_options (),
  with_stdout (s1, plot2d (sin(x), [x, 0, 1], [background_color, false],
                           [plot_format, xmaxima])),
  with_stdout (s2, plot2d (sin(x), [x, 0, 1], [plot_format, xmaxima])),
  r: [ssearch ("background", get_output_stream_string (s1)),
      integerp (ssearch ("{background #ffffff}",
                         get_output_stream_string (s2)))],
  reset_plot_options (),
  r);
[false, true]$

Commit message

plot2d, plot3d: omit a background color set to false

The gnuplot and xmaxima PLOT-PREAMBLE methods always wrote the
background color, and RGB-COLOR of false is NIL, so
[gnuplot_svg_background, false] gave set obj 1 fc rgb NIL for gnuplot
and {background NIL} for xmaxima. Write the background only when there
is a color, which leaves the default of the graphic program as
documented.

This fixes bug #5372.

Found and fixed by Claude Opus 5.5.