I explicitly ignore all lines that have '-' values

These may come from vnl-filter -p "dx=diff(x)" ... output. For instance, this
plot is broken before this patch:

  (echo '# x'; seq 5) | vnl-filter --noskipempty -p 'd=diff(x)' \
  | feedgnuplot --lines

changelog bump

changelog bump

version bump

feedgnuplot Debian release 1.60-1

version bump

feedgnuplot Debian release 1.58-1

changelog bump

added .gitignore

feedgnuplot Debian release 1.57-1