Add support for dynamic composition of plot series using higher-order observables

[glopesdev]

To allow cases where the number (and order) of plot series is not known beforehand, we could imagine a new overload for design visualizers that accepts a higher-order observable sequence (i.e. IObservable<IObservable<TData>>).

The behavior would be to create a new series every time a new observable sequence is received (and possibly delete it when the sequence completes?) and overlay it on the plot.