#1549 arrow with lt 0 is not straight on gif terminal

[Shigeharu TAKENO]

I found a problem of an arrow line on gif terminal for gnuplot 5.0,5.1.
The arrow line with "lt 0" is drawn like a hand-writing curve.

set term gif
set out 'test-gif.gif'
#set arrow from 1,0 to 0,1 lt 1 # <= no problem
set arrow from 1,0 to 0,1 lt 0 # <= as hand-writing curve
#plot [0:1][0:1] x w l lt 0 # <= no problem
plot [0:1][0:1] x w l lt 1 # <= no problem

We use gd library libgd-2.1.0 on Solaris 10 for both gnuplot 5.0 and 5.1.
The same problem also occurs on Win7 (wgnuplot-5.0.0), and may not
occurs on gnuplot 4.6.

[Ethan Merritt]

That is indeed a very strange error.

I don't know what to say except to repeat that useful dot/dash lines are not supported by libgd. We try to provide a dotted line for the axes (lt 0), but the axis is always a horizontal or vertical straight line. Curves and diagonals produce bad output. Your example is an extreme case of that. We might be able to improve it, but it will never be perfect unless libgd itself is improved.

[Shigeharu TAKENO]

Your example is an extreme case of that.

This problem also occurs in 3D grid.

I found this may be by the quantization error. Attached patch may fix it.

[Ethan Merritt]

Looks good!
Applied to 5.0 and 5.1

thanks