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You can download the matplotlib free from: http://matplotlib.sourceforge.net/

It has, amongst many other things, the module pylab that allows for high quality plotting of data. All you have to do is to feed it a list of x values and the corresponding list of y values calculated from the x data and you are ready to enjoy the plot in its separate window/frame. From there you can zoom the graph, drag the graph, change the margins, and save the graph to a popular image file like PNG.