How to Use
Enter a function f1(x,y) in the text input field marked "f1(x,y)=".

    Example: f1(x,y)= 5-x2-y2

Click the "Graph" button (this button also refreshes the graph).

Rotate the graph by clicking and dragging the mouse on the graph.

To see two functions graphed simultaneously:

Enter the second function f2(x,y) in the text input field marked "f2(x,y)=".

Click (to its "on" state) the check box next to this input field.

Click the "Graph" button

    Example: f1(x,y)= 5-x2-y2 and
      f2(x,y)=1-x-y

Up to three functions can be graphed simultaneously.

To remove a function from the graph, click (to its "off" state) the check box next to the associated text input field and click the "Graph" button to refresh.

To erase the graph and all input fields, click the "Clear" button.

Examples
 Plane: f1(x,y)=x-2y

 Hemisphere: f1(x,y)=(25-x2-y2)

 Paraboloid: f1(x,y)=x2+y2-8

 Saddle: f1(x,y)=(x2-y2)/10

 Waves: f1(x,y)=cos(x-y)

 Ripples: f1(x,y)=cos(2(x2+y2))

Other Notes
Surfaces in three dimensions are represented in "faceted hidden surface" form. The facets are not subdivided at intersections of surfaces if more than one surface is drawn, so intersections of surfaces are not precise. The "wireframe" representation for surfaces, in which the surface is transparent, only draws one surface at a time.

The graphs of the functions are color-coded, corresponding to the colors of the labels marking the text input fields: the graph of f1(x,y) is blue, f2(x,y) is red, and f3(x,y) is green.