Advantages: It's very flexible, scriptable, and freely available.
Disadvantages: A bit complex to learn (but I figured out how to get started in a few minutes, and quite functional in an hour or two), runs in a terminal window (if you consider that a disadvantage).
Something I found very useful was to script it to reload my terminal program's logfile periodically so i got a dynamic graph as my experiment progressed.
Edit: Here is the GnuPlot script that plots it:
#!/usr/local/bin/gnuplot -rv
# Note reverse video here ^^^ til I find a way to put it in the script
# gpFanCtl - Plots DiffThermo fan controller data (aloft, alow, Tdiff, fan-state).
# $Id: gpFanCtl,v 1.8 2014-04-28 09:40:51-04 jrobert Exp jrobert $
set terminal x11 1 noraise
set xtics 3600
set mxtics 4
set xdata time
set ytics 1 nomirror
set mytics 2
set y2range [0:3]
set y2tics 1
set my2tics 4
set grid
set ylabel 'Temperature, degC'
set y2label 'Tdiff, degC' textcolor rgb '#00CD63'
cd '/Users/jrobert/Desktop'
plot "Logfile.txt" using ($0*4):1 title "Aloft" with lines lc rgb "red",\
"Logfile.txt" using ($0*4):2 title "Alow" with lines lc rgb "#3982FF",\
"Logfile.txt" using ($0*4):3 title "Tdiff" with lines lc rgb "#00CD63" axis x1y2,\
"Logfile.txt" using ($0*4):4 title "Fan" with lines lc rgb "orange" axis x1y2;
pause 4
refresh
reread