OmniGraphSketcher for Mac

From Omni Group:

OmniGraphSketcher helps you make elegant and precise graphs in seconds, simply by sketching what you want. Specifically designed for reports, presentations, and problem sets where you need to produce sharp-looking graphs on the fly, OmniGraphSketcher combines the data plotting power of charting applications with the ease of a basic drawing program.
Add as many lines as you like to represent goals, cutoffs, or other boundaries. Shade in areas, connect and align objects, and add labels anywhere on the graph as you go. Enter data visually with a click of the mouse, and if your data is already in a spreadsheet or database program, just copy and paste it right into OmniGraphSketcher.
OmniGraphSketcher has an intuitive and simple interface so that you can quickly find what you want, while behind-the-scenes algorithms take care of all the details. It's an ideal solution for anyone who needs to make plots and graphs without the hassle of using a complicated data visualization program.