Gnuplot is is a plotting program that is available for both windows and unix/linux platforms. The main web site for the package is the Gnuplot information site.
From the "Gnuplot FAQ page"one reads that
"gnuplot is a command-driven interactive function plotting program. It can be used to plot functions and data points in both two- and three-dimensional plots in many different formats. It is designed primarily for the visual display of scientific data. gnuplot is copyrighted, but freely distributable; you don't have to pay for it. "
Gnuplot is useful for visualizing one and two dimensional data sets and can create line and surface plots in forms that are very similar to those created with Matlab. In addition to Gnuplot FAQ page as a reference, it is also useful to look at the gnuplot demonstration page.
The Gnuplot information site provides information on how to obtain the program. On Linux/Unix systems it is usually pre-installed. For windows, there is a binary (e.g. executable) version that one can download . To save you the bother of locating the windows download site, I've made a copy of gp423win32.zip that you can download by clicking on the link. All you need to do is unzip the file, and then click on the wgnuplot.exe file in the gnuplot/bin directory.