#data11.txt
#1	2	4
#2	4	4
#3	6	9
#4	8	16
#5	10	25
#6	12	36
#7	14	49
#8	16	64
#9	18	81
#10	20	100

set terminal png size 600, 400
set output 'plot.png'

#plot data col1:col2 if (19<col3 or col3>65)
plot  'data11.txt' u 1:($3<19 || $3>63  ? $2 : 1/0)  w lp ps 5