#5485 nognuplot_svg_background has no effect

[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/plot.lisp, PLOT-OPTIONS-PARSER

The keyword sets $gnuplot_svg_background, which nothing reads. [gnuplot_svg_background,c] stores its value in $background_color, so the keyword has to clear $background_color to be equivalent to [gnuplot_svg_background,false], as documented.

Note: Apply together with or after [#5372]. Without that fix, plotting with nognuplot_svg_background writes the same invalid gnuplot commands as [gnuplot_svg_background, false] does on master.

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 file src/plot.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 * \#5485 nognuplot_svg_background has no effect 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

set_plot_option(nognuplot_svg_background)$
get_plot_option(background_color);
reset_plot_options()$

Before the fix:

[background_color,"white"]

Correct result:

[background_color,false]

Fix

In src/plot.lisp (PLOT-OPTIONS-PARSER, line "($nognuplot_svg_background (setf"), replace

         ($nognuplot_svg_background (setf (getf options '$gnuplot_svg_background) nil))

with

         ($nognuplot_svg_background (setf (getf options '$background_color) nil))

Test cases

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

/* Bug #5485: "nognuplot_svg_background has no effect" */

block ([r],
  reset_plot_options (),
  set_plot_option (nognuplot_svg_background),
  r: get_plot_option (background_color),
  set_plot_option ([background_color, "white"]),
  reset_plot_options (),
  r);
[background_color, false]$

Commit message

plot.lisp: fix the nognuplot_svg_background option

The keyword nognuplot_svg_background set the option
$GNUPLOT_SVG_BACKGROUND, which nothing reads, while
[gnuplot_svg_background, c] stores its value in $BACKGROUND_COLOR. Clear
$BACKGROUND_COLOR, so that the keyword is equivalent to
[gnuplot_svg_background, false] as documented.

This fixes bug #5485.

Found and fixed by Claude Opus 5.5.