#1509 Plotting points with linetype nodraw gives wrong colors.

[Christoph Bersch]

Plotting points with lt nodraw selects a line color which depends on the position in the plot command.

Consider the following command:

set terminal pngcairo
set output 'points-nodraw.png'

set object rectangle from screen 0,0 to screen 1,1 fc rgb 'green' fillstyle solid noborder behind
set samples 11
unset tics

set multiplot layout 2,2
set title 'lt nodraw on 2.'
plot x with lines lt nodraw, x with points lt nodraw pt 7 ps 2
unset object

set title 'lt nodraw on 3.'
plot x with lines lt nodraw, x with lines lt nodraw, x with points lt nodraw pt 7 ps 2

set title 'lt nodraw'
plot x with points pt 7 ps 2 lt nodraw, 2 * x with lines lt nodraw lw 3

set title 'lt -2'
plot x with points pt 7 ps 2 lt -2, 2 * x with lines lt -2 lw 3

unset multiplot

I attached the output. Other terminals show the same behavior.

Shouldn't lt -2 and lt nodraw behave the same?

Another, related thought: shouldn't

plot x with points lt -2

skip the points? I see, that using e.g. plot x with linespoints lt nodraw might behave differently, but should that hold also when plotting only points?

[Ethan Merritt]

The first three panels in your example produce exactly the result I would have expected. "nodraw" is a linetype, not a color. The color still has to come from somewhere, and the usual case is to take it from the linetype matching the sequence in the plot. That is what happens here.

If you modify the plot commands to use
with linespoints lt nodraw lc 'cyan'
then the color will come from the lc keyword rather than from the plot sequence, but the linetype will still be "nodraw".

The idea was to provide keywords ("bgnd" "black" "nodraw") rather than the mysterious negative linetype numbers.

I don't know what's going on with the 4th panel (lt -2).

... checking ...

Aha. In misc.c line 1060

  new_lt = int_expression();
  load_linetype(lp, new_lt);

Maybe it should not do load_linetype for negative linetypes?
Or maybe it should reset the linetype afterwards?
I'm not sure, but in any case that is the point of difference between giving lt -2 and lt nodraw.

[Christoph Bersch]

I was still stuck at the point, that one could use lt inside set linetype, in which case the lt actually specified the color, and only that. I've seen now, that set linetype doesn't accept this anymore, good :)

Seeing this, I also think, that your solution to allow plotting only points with the linespoints plotting style, which you gave in #1374 4.65 Do not plot points when linewidth is set to 0 for linepoints doesn't work as proposed:

set linetype 1 linetype nodraw point pointtype 6
plot x with linespoints linetype 1

gives the error linetype definition cannot use linetype. So, the whole purpose of treating with points lt nodraw differently from with lines lt nodraw doesn't work that way.

And, what actually confused me was this special treatement of lt nodraw, which contradicts its intuitive meaning (draws points although it says nodraw). And the linetype contains the definition of points and lines, so why should it behave differently depending on the selected plotting style?