Bug#901614: gnuplot: clipped line segments disappear

[b606]

Source: gnuplot
Severity: important



I can reproduce this bug with the gnuplot version in debian stretch 
5.0.5+dfsg1-6+deb9u1
and the latest gnuplot 5.2.4 compiled from source, using set terminal
pngcairo, svg or epslatex_standalone.


1) Draw a triangle within xrange and yrange
   Everything is OK. (see result of gnuplot -c triangle.gnuplot)
   
2) Reduce x or y ranges -> some borders disappear  (see file 
triangle_partial.gnuplot)

3) All borders disappear when both ends of the line segments are out of range,
   resulting in an empty figure.
   For example: set the plot ranges in the gnuplot files to
   plot [-0.75000000 : 0.75000000] [-0.75000000 : 1.25000000] '-' notitle with 
lines lt rgb "#0000ff"
   
Fortunately enough, the filledcurves are OK so that 
one can check where these lines should be drawn 
(see file triangle_partial_filled.gnuplot, in light blue).

Expected behaviour: draw partial line segments if within viewport ranges.



-- System Information:
Debian Release: 9.4
  APT prefers stable
  APT policy: (500, 'stable')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 4.9.0-6-amd64 (SMP w/4 CPU cores)
Locale: LANG=fr_FR.UTF-8, LC_CTYPE=fr_FR.UTF-8 (charmap=UTF-8), LANGUAGE=fr 
(charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

set terminal pngcairo solid background "white" enhanced font "arial,10" 
fontscale 1.0 size 600,400; set zeroaxis;; 
set term pngcairo
set output "triangle_partial.png"
unset colorbox
set xlabel "x"
set ylabel "y"
set grid front
unset grid
set size ratio 1.00000000
set xrange [-1.25000000 : 1.25000000]
set yrange [-1.25000000 : 1.25000000]
set zeroaxis
set datafile missing "NaN"
plot [-0.75000000 : 1.25000000] [-0.75000000 : 1.25000000] '-' notitle with 
lines lt rgb "#0000ff"
-1. 1. 
1. -1. 
1. 1. 
-1. 1. 

set terminal pngcairo solid background "white" enhanced font "arial,10" 
fontscale 1.0 size 600,400; set zeroaxis;; 
set term pngcairo
set output "triangle.png"
unset colorbox
set xlabel "x"
set ylabel "y"
set grid front
unset grid
set size ratio 1.00000000
set xrange [-1.25000000 : 1.25000000]
set yrange [-1.25000000 : 1.25000000]
set zeroaxis
set datafile missing "NaN"
plot [-1.25000000 : 1.25000000] [-1.25000000 : 1.25000000] '-' notitle with 
lines lt rgb "#0000ff"
-1. 1. 
1. -1. 
1. 1. 
-1. 1. 

set terminal pngcairo solid background "white" enhanced font "arial,10" 
fontscale 1.0 size 600,400; set zeroaxis;; 
set term pngcairo
set output "triangle_partial_filled.png"
unset colorbox
set xlabel "x"
set ylabel "y"
set grid front
unset grid
set size ratio 1.00000000
set xrange [-1.25000000 : 1.25000000]
set yrange [-1.25000000 : 1.25000000]
set zeroaxis
set datafile missing "NaN"
plot [-0.75000000 : 1.25000000] [-0.75000000 : 1.25000000] '-' notitle with 
filledcurves lc rgb '#d7e3f4'
-1. 1. 
1. -1. 
1. 1. 
-1. 1.