gnuplot conditional plotting: plot col A:col B if col C == x

Consider the following dataset (1.dat),

1 0.8 0
2 0.6 0
3 0.9 1
4 1.1 0
5 0.7 0
6 0.6 1

where we want to plot the first two columns only when the third one equals zero. Then you can try this:

plot '1.dat' using 1:($3==0?$2:1/0)

(Credit to markjoe on Gnuplot mailing-list.)