set title "Sample points for cos(6*pi*t)\nWith a period of 1/3"
set xlabel "Time (t)"
set ylabel "f(t) = cos(6*pi*t)"
set pointsize 1
plot [x=0:2] [-1:1] "four11.txt" every :::0::0 using 1:2 title "Once every second" with linespoints, \
"four11.txt" every :::2::2 using 1:2 title "Once evey half second" with linespoints, \
"four11.txt" every :::4::4 using 1:2 title "Once evey quarter second" with linespoints, \
"four11.txt" every :::6::6 using 1:2 title "Once evey eigth of a second" with linespoints, \
"four11.txt" every :::8::8 using 1:2 title "Once evey sixteenth of a second" with linespoints