The trigger onset function gives you “time” in terms of number of samples into the data, so you have to convert that back to actual time again. So something like the following based on your code snippet
events = []
for on, off in trigList:
t = tr.stats.starttime + tr.stats.delta * on
events.append({'time': t, 'text': ''})
tr.plot(type="dayplot", events=events)