It is quite simple:
set multiplot layout 1,2 plot '-' u 1:2 1 2 2 3 3 4 e plot '-' u 1:2 2 3 3 4 4 5 e
Please note that the embedded data is actually not very happy with the pseudo file. '' At this point, you really need to include all of your data. So, if you want 2 tracks on the same subset of the multiplier:
set multiplot layout 1,2 plot '-' u 1:2, '-' u 1:3 1 2 3 4 5 6 7 8 9 e 1 2 3 4 5 6 7 8 9 e plot '-' u 1:($2*$3) 1 2 3 4 5 6 7 8 9 e
This ends up being the same as if you had a data file data.txt :
#data.txt 1 2 3 4 5 6 7 8 9
and built it using this (much simpler) script:
set multiplot layout 1,2 plot 'data.txt' u 1:2, '' u 1:3 plot '' u 1:($2*$3)
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