From time to time I need graphs for tests, or index cards, or to display on screen. In the past I used TI-InterActive! which is a Windows-only app that mimics a TI-84's graphing ability. The resolution and clarity is about the same too. I have discussed many other math tools out there, but I've got one that so far that nets the prettiest results.
OmniGraphSketcher is a $30 app that lets you freeform any kind of graph. The intent is generalized graphics for business presentations, but you can use them for math too. You define the points, determine whether they connect with straight or curved sections, and can adjust the points by hand if necessary. Tons of formatting options for pretty axes, large grid squares, labels, shading etc. The "Copy as Image" feature generates items good enough for printing. Though draw a graph much larger than you need, it's prettier to scale down than up. If anything it's also fabulous when you need a blank, easy to read grid for a test.