#!/bin/sh
# Make gnuplot fill the legend box - but only when the legend is inside the axes.
#
# Octave's gnuplot backend emits `set key ...` but never `set key opaque`, and
# gnuplot's default is noopaque - so a legend drawn INSIDE the axes has a
# transparent background and plot lines run straight through the text. On a
# busy plot the legend becomes unreadable, which is exactly when a legend
# matters most. The legend axes already has color [1 1 1]; Octave simply never
# tells gnuplot to fill it.
#
# An OUTSIDE key needs the opposite. gnuplot packs a key below the axes flush
# against the x-axis label - a one-pixel gap, whatever the type size - so the
# fill lands on the label's descenders and erases the tail of every g, y and p
# in it: "from the trigger edge" renders as "from the triaaer edae". Out there
# the key sits on the white margin with no plot lines to hide behind it, so
# the fill buys nothing and costs the label. `noopaque` is also stated rather
# than left to the default, because `set key opaque` set by an earlier axes
# persists through the rest of a multiplot stream.
#
# Done at image build time rather than at runtime because it edits a vendored
# Octave library file: in the Dockerfile it is reproducible, visible in review,
# and cannot be half-applied. It exits 0 when the anchor is absent, so an
# Octave upgrade that moves this code degrades to the old rendering instead of
# failing the build.
set -e

ANCHOR='__do_enhanced_option__ (enhanced, hlgnd));'
ADDED='owuitools: gnuplot defaults to a transparent key'
found=0

for f in /usr/share/octave/*/m/plot/util/private/__gnuplot_draw_axes__.m; do
  [ -f "$f" ] || continue
  if grep -q "$ADDED" "$f"; then
    echo "legend-opaque: already applied to $f"
    found=1
    continue
  fi
  n=$(grep -c "$ANCHOR" "$f" || true)
  if [ "$n" != "1" ]; then
    echo "legend-opaque: anchor appears $n times in $f, skipping"
    continue
  fi
  awk -v anchor="$ANCHOR" -v added="$ADDED" '
    { print }
    index($0, anchor) && !done {
      done = 1
      print "    ## " added ", so plot lines show"
      print "    ## through the legend text. The legend axes is already white."
      print "    ## Outside the axes the fill has nothing to hide and lands on"
      print "    ## the x-axis label instead, so there it is turned off - and"
      print "    ## said out loud, because opaque persists across a multiplot."
      print "    if (strcmp (inout, \"inside\"))"
      print "      fputs (plot_stream, \"set key opaque;\\n\");"
      print "    else"
      print "      fputs (plot_stream, \"set key noopaque;\\n\");"
      print "    endif"
    }
  ' "$f" > "$f.owuitools" && mv "$f.owuitools" "$f"
  echo "legend-opaque: applied to $f"
  found=1
done

[ "$found" = "1" ] || echo "legend-opaque: target not found, skipping"
exit 0