Graphique is a graphing calculator for OS X Lion that was developed in conjunction with the book Beginning OS X Lion Apps Development (Apress 2011). It features a smart, syntax-highlighting editor, results in both graph and table format, a QuickLook plugin, and printing capabilities. Complete source code is available at the Apress site.
Buy the app so you can visualize your equations. Buy the book so you can understand how to build OS X Lion applications!