1. Select "Custom Function Plots" from either the Graphs|2D Graphs or the Graphs|3D XYZ Graphs submenu 

2. Specify the function in the respective dialog.

Also, you can add a custom function plot to any existing graph by the following steps:

1. Access the Custom Function options pane of the Graph Options dialog and click the Add new function button.

2. Use the options in the Custom Function options pane to specify the equation to be plotted in the 2D or 3D graph.

In addition to the standard math functions, a variety of functions representing distributions as well as their integrals and inverses are supported and can be plotted (including Beta, binomial, Cauchy, Chi-square, exponential, F, Gamma, geometric, Laplace, logistic, normal, log-normal, Pareto, Poisson, Student's t, and Weibull distributions).