Plotting lines with missing datapoints for multidimensional data - gnuplot

Question

I'm trying to plot multiple lines representing GPU usage over time from a dataset which records data of multiple GPUs. Each row contains the timestamp, a GPU index and the usage in percent.
My dataset looks like this:
$ cat gpu.txt
#time #index # usage (%)
1,1,10
1,2,5
2,1,20
2,2,10
3,1,40
3,2,30
and this is my gnuplot script:
$ cat plot.gplot
set datafile separator ","
set autoscale # scale axes automatically
unset log # remove any log-scaling
unset label # remove any previous labels
set xtic auto # set xtics automatically
set ytic auto # set ytics automatically
set title
set term png
set title "GPU usage"
set xlabel "Time"
set ylabel "Usage"
set output "gpu.png"
plot "gpu.txt" using ($2 == 1 ? $1 : NaN):($2 == 1 ? $3 : NaN) title 'GPU1' with linespoints ls 10 linecolor rgb "blue", \
"gpu.txt" using ($2 == 2 ? $1 : NaN):($2 == 2 ? $3 : NaN) title 'GPU 2' with linespoints ls 10 linecolor rgb "red", \
Unfortunately, this only ever draws the singular datapoints, but no lines. I think this is because of "missing" datapoints - which is not the case obviously because I have the custom filters in place to plot usage data per GPU index. I tried to indicate this to gnuplot via the NaN value, but it doesn't seem to work.
Example output:

Answer 1

This is kind of a recurring filtering data question.
You can define linestyles and then use it in the plotting loop via ls i.
Essential if you want connecting lines is the line: set datafile missing NaN.
My minimal suggestion would be:
Code:
### filtering data
reset session
$Data <<EOD
#time #index # usage (%)
1,1,10
1,2,5
2,1,20
2,2,10
3,1,40
3,2,30
EOD
set datafile separator ","
set title "GPU usage"
set xlabel "Time"
set ylabel "Usage"
set key top left
set datafile missing NaN
myFilter(datacol,filtercol,value) = value==column(filtercol) ? column(datacol) : NaN
plot for [i=1:2] $Data u (myFilter(1,2,i)):3 w lp pt 7 title sprintf('GPU%d',i)
### end of code
Result: