Gnuplot is present on almost every Unix or Linux system and is thus the choice for a quick plot of whatever data you need to visualize.

Basic premises:

Start gnuplot and plot your file:

  $ gnuplot
  gnuplot> plot "filename.dat" with lines

A plot should appear. If there are two columns in the file, you can assign these to the X and Y axis with the 'using' command:

  gnuplot> plot "filename.dat" using 1:2

'using 1:2' tells the plot command to use the first column as the x-axis and the second as the y axis.

To plot multiple datafiles:

  gnuplot>  plot "file1.dat" with lines,  "file2.dat" with lines