Overlapping events are hard to read in TimeSeries.plot()

[felixchenier]

Overlapping events are superposed in TimeSeries.plot(), which makes it difficult to read. This is a somewhat minor issue, but if they share an exact same time, it is impossible to read their name correctly even in zooming since they will always be drawn one on top of the other. It would be nice to check for these conditions and write event names one below the other in these cases.