Let's suppose my dataset is like this:
0 0.3
1 0.12
2 0.4
3 0.6
4 0.9
...
10 0.23
11 0.6
...
20 0.34
21 0.4
...
and I'd like to plot values of both columns only if $1 % 10 == 0, i.e., (0,0.3), (10,0.23), (20,0.34) and so on... Now, I've written the following conditional script:
plot "data.csv" using 1:(int($1)%10==0?$2:0/0) title 'r=1' with linespoints linewidth 3 linecolor rgb 'blue'
The problem is that lines are not shown, but only points.

This is because, for all rows where the condition is not satisfied, the corresponding value is undefined. Anyway, what I need is quite different; I want those specific values to be just ignored, not to set to undefined. Is there a way to do that just using gnuplot (not awk and so on)?