#5482 plot2d with logy warns that values will be clipped when none are

[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, DRAW2D

With logy, DRAW2D compares the logarithm of each y value against the linear bounds of the y range, and only afterwards applies EXP. So values inside the range are counted as clipped.

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 * \#5482 plot2d with logy warns that values will be clipped when none are 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(x^2,[x,1,10],[y,0.5,200],logy,[plot_format,gnuplot],[run_viewer,false],[gnuplot_script_file,"/tmp/b28.gp"]);

Before the fix:

plot2d: some values will be clipped.

Correct result:

no clipping message

Fix

In src/plot.lisp (DRAW2D, loop "Fix up out-of-range values", line "(if (numberp (car y))"), replace

            ((null y))
          (if (numberp (car y))

with

            ((null y))
          (when (and (getf plot-options '$logy)
                     (numberp (car y)))
            (setf (car y) (exp (car y))))
          (if (numberp (car y))

In src/plot.lisp (DRAW2D, same loop, block "(when (and (getf plot-options '$logy)" at its end), replace

            (setf (car x) (exp (car x))))

          (when (and (getf plot-options '$logy)
                     (numberp (car y)))
            (setf (car y) (exp (car y)))))

with

            (setf (car x) (exp (car x)))))

Test cases

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

/* Bug #5482: "plot2d with logy warns that values will be clipped when none are" */

block ([gpfile: sconcat ("rtest_plotoptions_", ?getpid (), ".gnuplot"),
        s1: make_string_output_stream (), s2: make_string_output_stream (),
        r],
  reset_plot_options (),
  with_stdout (s1, plot2d (x^2, [x, 1, 10], [y, 0.5, 200], logy,
                           [plot_format, gnuplot], [run_viewer, false],
                           [gnuplot_script_file, gpfile])),
  with_stdout (s2, plot2d (x^2, [x, 1, 10], [y, 1, 5], logy,
                           [plot_format, gnuplot], [run_viewer, false],
                           [gnuplot_script_file, gpfile])),
  r: [ssearch ("clipped", get_output_stream_string (s1)),
      integerp (ssearch ("clipped", get_output_stream_string (s2)))],
  reset_plot_options (),
  r);
[false, true]$

Commit message

plot2d: fix spurious clipping message with logy

With logy, DRAW2D compared the logarithm of each y value with the linear
bounds of the y range and applied EXP only afterwards, so values inside
the range were reported as clipped. Apply EXP before the comparison.

This fixes bug #5482.

Found and fixed by Claude Opus 5.5.