Plotly cleanup

This is a small cleanup and to fix a FIXME and separate the plotly-related code.

The FIXME was related to plotting the imaginary part of a waveform. Now the imaginary is only plotted if the dtype of the numpy array is complex.

A second fix removed a weird dummy for that seemed to be there just because the hover functionality of plotly was not being used correctly.

And last, makes no sense keep the plotly stuff under the matplotlib utilities.

See individual commits for comprehensive diffs (because the code was moved after fixed)