GNUPLOT

Load data from file.name and use a spreadsheet-like command:

• plot "file.name" using 5:($6*2+158) with [lines, dots, points]

The annotation will replace the raw commands appeared in the legend

• plot "file.name " with [lines, dots, points] title "annotation"
  

Use awk, use "< awk '{whatever}' " in the place of filename. Note there is no space after the 1st quotation.

make a plot on top of another one (like hold on function in MATLAB)

• set multiplot on
• unset multiplot

make 3 plots in one figure (like subplot in MATLAB)

• set multiplot on
• set size 0.5, 0.5 (default is 1, 1. This makes it 25% smaller)
• set origin 0,0 (default)
• plot whatever
• continue use set size/origin until it is done.

output to a file

• set output "filename.png"
• set terminal png (other options include: set term postscript color)
  
• replot
  

Good tutorials can be found in the following:

• T. Kawano -- not so much FAQ, which includes how to plot special symbols or Greek letters