Not terribly surprising, considering that you're going to get a lot of complex function values. (-1/2)^(1/2) is complex, at least. Try plotting just the magnitude, or just the real part, or something like that instead.
In order that $\displaystyle a^x$ be real (and singlevalued) a must be larger than or equal to 0. If a< 0, you will get complex values as Ackbeet said. You cannot graph $\displaystyle y= a^x$ on real axes for a< 0.