Texas Instruments TITANIUM TI-89 - Manual (Page 450)

Differential Equation Graphing 450
Fields=DIRFLD Fields=DIRFLD Fields=DIRFLD Fields=DIRFLD
In the
Y= Editor Enter a valid system of two 1st-order equations. For
information about defining a valid system for a 2nd-order
equation, refer to Example of a 2nd-Order Equation.
Set Axes = CUSTOM:
@ 2
‰ If Axes = TIME, an Invalid axes error occurs
when you graph.
If you enter initial conditions in the Y= Editor, the equations
referenced by the custom axes must have the same number
of initial conditions.
Otherwise, a Dimension error occurs when you graph.
With custom
axes Set axes that are valid for your system of equations.
Do not select t for either axis. Otherwise, an Invalid axes error
occurs when you graph.
The two axes must refer to different equations in your system
of equations. For example, y1 vs. y2 is valid, but y1 vs. y1'
gives an Invalid axes error.