If you're using a linux/unix platform, you probably already have Gnuplot installed. Gnuplot can take a plot file and a datafile and generate an image, thus:
History.gnuplot, which needs to be generate first:
set title "Rawr_satch history"
set xlabel "Time"
set ylabel "Ranking"
set output "rawr_satch_graph.png"
set terminal png color
set xdata time
plot "rawr_satch_xphistory.dat" with linespoints
Which assumes a data file rawr_satch_xphistory.dat
has already been generated, formatted thus:
2010-03-06 385581123
2010-03-05 384895430
2010-03-04 382983388
People have also written interfaces into Gnuplot for most languages, for example PHP-GNUPlot. Gnuplot scripts can be quite complex, and you could plot multiple variables, etc.