Automatically add labels, titles and grids to plotcoeffs plots.

Closes #1272.

In the Chebyshev case, we have:
* Title: Chebyshev coefficients
* x-label: Degree of Chebyshev polynomial
* y-label: Magnitude of coefficient

In the Fourier case, we have
* Title: Fourier coefficients
* x-label: Frequency of complex exponential
* y-label: Magnitude of coefficient