Graph of natural log of x, for negative x is incorrect

[hackerb9]

The underlying math library in calculator++ correctly returns complex numbers for $\ln(x)$, where $x<0$. For example,

$$\ln(-5) = \ln(5) + \pi i$$

However, the graphing part ignores complex numbers and so the graph is displayed as symmetric, $\ln(-x) = \ln(x)$, which is incorrect.

While this could be fixed by resolving issue #210, a simpler solution is to treat all numbers where the complex part is not zero as undefined, which is the traditional way logarithms were handled mathematically.