Can you determine the sign of the polar form of a complex number?

[schaefer.mp at gmail.com]

Just to clarify what I'm after:

If you plot (-3)^n where n is a set of negative real numbers between 0
and -20 for example, then you get a discontinuos line due to the
problem mentioned above with fractional exponents. However, you can
compute what the correct absolute value of the the missing points
should be (see z2 above for an example), but I would like to know how
to determine what the correct sign of z2 should be so that it fits the
graph.