Mac Grapher is a graphing tool available on every Mac, and it makes great graphs. This video describes how you can customize your Grapher graphs to have better labels, more colors, different shapes, etc.
Some "great graph" tips to keep in mind:
* use well-chosen colors for each curve or set of points
* don't make your curves too thin
* label the curves and the axes, using arrows if needed
* make a title box for the final result that summarizes the content
* plot on the appropriate axes (e.g., logarithmic, linear)
* don't overdo the background
* write a good caption once you embed the final graph
