Simple expressions can be entered on the command line. Igor graphs automatically update when their data is changed.
Arbitrarily complex functions, including conditionals and loops, can be expressed as user-defined functions using Igor's built-in programming language. Simple command-line assignments can reference your functions.
A Function Grapher is provided as an aid to creating a function and displaying it in a graph:
Igor Pro 9
Igor XOP Toolkit
Igor NIDAQ Tools MX