Tests fail on Gentoo with gnuplot 6.0.2 (ERROR: the gnuplot backend issued an error: line 0: warning: Invalid terminal font size)

Reproduce the failure with gnuplot 6.0.2 by running the test suite, then start with t/aa.t and the Gnuplot.pm locations reported in the traceback. Compare the terminal-gif test with the version-related checks in t/plot.t; done means the tests complete successfully without the reported backend error.

Tests fail on Gentoo with gnuplot 6.0.2:

huettel@kona ~/Gentoo/gentoo/dev-perl/PDL-Graphics-Gnuplot $ ebuild PDL-Graphics-Gnuplot-2.32.0.ebuild clean test
Appending /home/huettel/Gentoo/gentoo to PORTDIR_OVERLAY...
Forcing test.
 * PDL-Graphics-Gnuplot-2.032.tar.gz BLAKE2B SHA512 size ;-) ...                                                                                                                            [ ok ]
>>> Unpacking source...
>>> Unpacking PDL-Graphics-Gnuplot-2.032.tar.gz to /var/tmp/portage/dev-perl/PDL-Graphics-Gnuplot-2.32.0/work
>>> Source unpacked in /var/tmp/portage/dev-perl/PDL-Graphics-Gnuplot-2.32.0/work
>>> Preparing source in /var/tmp/portage/dev-perl/PDL-Graphics-Gnuplot-2.32.0/work/PDL-Graphics-Gnuplot-2.032 ...
>>> Source prepared.
>>> Configuring source in /var/tmp/portage/dev-perl/PDL-Graphics-Gnuplot-2.32.0/work/PDL-Graphics-Gnuplot-2.032 ...
 * Using ExtUtils::MakeMaker
 * perl Makefile.PL AR=x86_64-pc-linux-gnu-ar CC=x86_64-pc-linux-gnu-gcc CPP=x86_64-pc-linux-gnu-gcc -E LD=x86_64-pc-linux-gnu-gcc NM=x86_64-pc-linux-gnu-nm RANLIB=x86_64-pc-linux-gnu-ranlib OPTIMIZE=-O2 -pipe -march=native LDFLAGS=-Wl,-O1 -Wl,--as-needed -Wl,-z,pack-relative-relocs PREFIX=/usr INSTALLDIRS=vendor INSTALLMAN3DIR=none
Checking if your kit is complete...
Looks good
'CPP' is not a known MakeMaker parameter name.
'NM' is not a known MakeMaker parameter name.
Generating a Unix-style Makefile
Writing Makefile for PDL::Graphics::Gnuplot
Writing MYMETA.yml and MYMETA.json
>>> Source configured.
>>> Compiling source in /var/tmp/portage/dev-perl/PDL-Graphics-Gnuplot-2.32.0/work/PDL-Graphics-Gnuplot-2.032 ...
make -j13 -l24 'OTHERLDFLAGS=-Wl,-O1 -Wl,--as-needed -Wl,-z,pack-relative-relocs' 'OPTIMIZE=-O2 -pipe -march=native' 
cp lib/PDL/Demos/Gnuplot.pm blib/lib/PDL/Demos/Gnuplot.pm
cp lib/PDL/Graphics/Gnuplot.pm blib/lib/PDL/Graphics/Gnuplot.pm
>>> Source compiled.
>>> Test phase: dev-perl/PDL-Graphics-Gnuplot-2.32.0
 * Test::Harness Jobs=13
make -j13 -l24 test TEST_VERBOSE=0 
PERL_DL_NONLAZY=1 "/usr/bin/perl" "-MExtUtils::Command::MM" "-MTest::Harness" "-e" "undef *Test::Harness::Switches; test_harness(0, 'blib/lib', 'blib/arch')" t/*.t
===(       1;0  1/4  0/?  0/? )=========================================PDL::Graphics::Gnuplot: ERROR: the gnuplot backend issued an error:
line 0: warning: Invalid terminal font size
 at /var/tmp/portage/dev-perl/PDL-Graphics-Gnuplot-2.32.0/work/PDL-Graphics-Gnuplot-2.032/blib/lib/PDL/Graphics/Gnuplot.pm line 7681.
        PDL::Graphics::Gnuplot::_checkpoint(PDL::Graphics::Gnuplot=HASH(0x55ab4054d4a0), "main", HASH(0x55ab4054ca50)) called at /var/tmp/portage/dev-perl/PDL-Graphics-Gnuplot-2.32.0/work/PDL-Graphics-Gnuplot-2.032/blib/lib/PDL/Graphics/Gnuplot.pm line 3314
        PDL::Graphics::Gnuplot::plot(undef, "with", "lines", PDL=SCALAR(0x55ab4054cbb8), PDL=SCALAR(0x55ab4054d398)) called at t/aa.t line 34
        main::__ANON__() called at /usr/lib64/perl5/vendor_perl/5.40/Test/Builder.pm line 368
        eval {...} called at /usr/lib64/perl5/vendor_perl/5.40/Test/Builder.pm line 368
        Test::Builder::subtest(Test::Builder=HASH(0x55ab3ecee110), "Terminal gif", CODE(0x55ab3eca0608)) called at /usr/lib64/perl5/vendor_perl/5.40/Test/More.pm line 831
        Test::More::subtest("Terminal gif", CODE(0x55ab3eca0608)) called at t/aa.t line 39
# Looks like your test exited with 255 just after 1.
t/aa.t ............... Dubious, test returned 255 (wstat 65280, 0xff00) 
Failed 3/4 subtests 
===(       2;0  0/?  1/? )==============================================# 
# P::G::G v2.032, gnuplot v6.0.2, Perl v5.040002, /usr/bin/perl on linux #
t/pdfcairo-output.t .. ok                                               
t/plot.t ............. 12/? Argument "6.0.2" isn't numeric in numeric lt (<) at /var/tmp/portage/dev-perl/PDL-Graphics-Gnuplot-2.32.0/work/PDL-Graphics-Gnuplot-2.032/blib/lib/PDL/Graphics/Gnuplot.pm line 4161.
t/plot.t ............. 35/? Argument "6.0.2" isn't numeric in numeric ge (>=) at t/plot.t line 183.
Argument "6.0.2" isn't numeric in numeric lt (<) at /var/tmp/portage/dev-perl/PDL-Graphics-Gnuplot-2.32.0/work/PDL-Graphics-Gnuplot-2.032/blib/lib/PDL/Graphics/Gnuplot.pm line 4177.
t/plot.t ............. 52/? Argument "6.0.2" isn't numeric in numeric eq (==) at t/plot.t line 255.
Argument "6.0.2" isn't numeric in numeric lt (<) at t/plot.t line 352.
INFO: Plotting to 'Plot-1.txt'
 at t/plot.t line 509.
INFO: Plotting to 'Plot-1.txt'
 at t/plot.t line 520.
INFO: Plotting to 'Plot-1.txt'
 at t/plot.t line 528.
# ******************************
# Skipping 27 interactive tests.
#     Set the environment variable GNUPLOT_INTERACTIVE to enable them.
# ******************************
Argument "6.0.2" isn't numeric in numeric lt (<) at t/plot.t line 756.
t/plot.t ............. 178/? Gnuplot warning: empty y range [0:0], adjusting to [-1:1]
 at t/plot.t line 881.
INFO: Plotting to 'Plot-1.txt'
 at t/plot.t line 986.
INFO: Plotting to 'Plot-2.txt'
 at t/plot.t line 989.
t/plot.t ............. ok     

Test Summary Report
-------------------
t/aa.t             (Wstat: 65280 (exited 255) Tests: 1 Failed: 0)
  Non-zero exit status: 255
  Parse errors: Bad plan.  You planned 4 tests but ran 1.
Files=3, Tests=221,  6 wallclock secs ( 0.06 usr  0.01 sys +  3.05 cusr  0.96 csys =  4.08 CPU)
Result: FAIL
Failed 1/3 test programs. 0/221 subtests failed.
make: *** [Makefile:885: test_dynamic] Error 255
 * ERROR: dev-perl/PDL-Graphics-Gnuplot-2.32.0::gentoo failed (test phase):
 *   emake failed
 * 

Gnuplot version is:

huettel@kona ~/Gentoo/gentoo/dev-perl/PDL-Graphics-Gnuplot $ gnuplot --version
gnuplot 6.0.2 patchlevel 2 (Gentoo revision r0)