Gnuplot

I use gnuplot from time to time. I don't know it well enough to do anything fancy, but I keep it in my toolbox. Most of the time I just want to plot some points from a data file.

[gnuplot documentation]

set terminal png 
#gif small size 640,480
set title "Daily clock offset\nnegative means clock runs fast"
set data style fsteps
set xlabel "Date"
set timefmt "%d/%m/%y\t%H%M"
set yrange [ -4.6 : -4.8]
set xdata time
set xrange [ "25/06/00":"29/08/00" ]
set ylabel "Required offset (negative minutes fast)"
set format x "%d/%m"
#\n%H:%M"
set grid
set key left
plot "data.dat" using 1:3
reset