I just came across a weird behaviour of the plot() function in the latest ObsPy release (0.8.4). Once I specify starttime and/or endtime on the plot() function, the stream that is plotted is actually altered and cut to that starttime & endtime. I'm not sure that this is working as intended, hence I report it here.
e.g. using stream.plot(starttime=t1, endtime=t2) will cut the data of stream so it only contains data from t1 to t2.