/usr/share/plplot_octave/support/ is in octave-plplot 5.9.9-2ubuntu2.
This file is owned by root:root, with mode 0o755.
..
/usr/share/plplot_octave/support/__comet.m
/usr/share/plplot_octave/support/__pl_contour.m
/usr/share/plplot_octave/support/__pl_draw_legend.m
/usr/share/plplot_octave/support/__pl_fill.m
/usr/share/plplot_octave/support/__pl_init.m
/usr/share/plplot_octave/support/__pl_logplotit.m
/usr/share/plplot_octave/support/__pl_matstr.m
/usr/share/plplot_octave/support/__pl_mesh.m
/usr/share/plplot_octave/support/__pl_meshplotit.m
/usr/share/plplot_octave/support/__pl_opt.m
/usr/share/plplot_octave/support/__pl_plenv.m
/usr/share/plplot_octave/support/__pl_plot3.m
/usr/share/plplot_octave/support/__pl_plotit.m
/usr/share/plplot_octave/support/__pl_polargrid.m
/usr/share/plplot_octave/support/__pl_store.m
/usr/share/plplot_octave/support/__plr1__.m
/usr/share/plplot_octave/support/__plr2__.m
/usr/share/plplot_octave/support/__plr__.m
/usr/share/plplot_octave/support/__plt1__.m
/usr/share/plplot_octave/support/__plt2__.m
/usr/share/plplot_octave/support/__plt2mm__.m
/usr/share/plplot_octave/support/__plt2mv__.m
/usr/share/plplot_octave/support/__plt2ss__.m
/usr/share/plplot_octave/support/__plt2vm__.m
/usr/share/plplot_octave/support/__plt2vv__.m
/usr/share/plplot_octave/support/__plt__.m
/usr/share/plplot_octave/support/is_strmatrix.m
/usr/share/plplot_octave/support/is_strvector.m