"draw" kwarg for waveform plotting

makes it possible to switch off the drawing the canvas after the plot
command. this makes it possible to do further adjustments to a plot
performed in an already existing window before refreshing the window
manually then.
waveform plot: make explicite "fast" method possible

a manually specified "method='fast'" was overridden by a check of number
of pointy in the data in some cases (e.g. mine ;)).