In this demonstration, we're shown how users can manipulate objects on a 3D plane via the Z-axis by pulling the pen away from the display.

ENGADGET: Hands-on with wireless, ultrasonic stylus and touchless gestures at MWC (video)

Complex numbers, which have a real and an imaginary part added together, are the points on this plane that do not lie on either axis.

ECONOMIST: Beno?t Mandelbrot

And, as Euler's successor, Carl Friedrich Gauss, was to discover, if you plot real numbers on one axis of a graph and imaginary ones on the other, you create a plane that represents both sorts of numbers.

ECONOMIST: Beno?t Mandelbrot