[Gnuplot-info] Disabling the automatic isolines of splot?

[Bjorn B. <bjo...@gm...>]

Hi all,

I have a question about the feature of `splot` that “If all blocks
contain the same number of points, gnuplot will draw cross-isolines
between points in the blocks, connecting corresponding points.” In
particular, I wonder if that feature can be turned off.

In my case, I have equally sized block that I want to plot and
individual lines, and not have them connected with a surface. This
example illustrates the problem:

```
set angles degrees
set view 70,140
set isotropic
set mapping spherical
set hidden3d offset 0

# Set up the Earth grid
set parametric
set samples 32
set isosamples 37
set urange [-90:90]
set vrange [0:360]

set style data lines
set zlabel "Earth radii" rotate

$DATA << EOD
# Orbit 1
 0   0 2
 0  30 2
 0  60 2
 0  90 2
 0 120 2
 0 180 2
 0 210 2
 0 240 2
 0 270 2
 0 300 2
 0 330 2
 0 360 2

# Orbit 2
30   0 2
30  30 2
30  60 2
30  90 2
30 120 2
30 180 2
30 210 2
30 240 2
30 270 2
30 300 2
30 330 2
30 360 2

# Orbit 3
60   0 2
60  30 2
60  60 2
60  90 2
60 120 2
60 180 2
60 210 2
60 240 2
60 270 2
60 300 2
60 330 2
60 360 2
EOD

# The for loop is necessary to prevent iso grid lines being drawn by
# gnuplot due to regular data block sizes.
splot \
  cos(u*2)*cos(v),cos(u*2)*sin(v),sin(u*2) lc "dark-grey" notitle ,\
  $DATA
```

The workaround I have is to plot the lines in a for loop and use
`every`. It is OK here in the example but quite a bit messier in my
“production” code:

```
splot \
  cos(u*2)*cos(v),cos(u*2)*sin(v),sin(u*2) lc "dark-grey" notitle ,\
    for [j = 0:2] $DATA every :::j::j
```

There was a similar question in 2015 which seems to have not been
answered: https://sourceforge.net/p/gnuplot/mailman/gnuplot-info/thread/5523D38B.2010001%40lpccaen.in2p3.fr/#msg33740382

Many thanks,
Bjorn