errorbars: memory use

If i have lots and lots of events and try and show them every event, i seem to get increasing memory use (filled 64gb) and i think this is because the remove() doesn't seem to work on the errorbar container - not sure as I haven't looked into it with a debugger.

to reproduce make one of the live dispatchers send 100 events in a for loop instead of just one and try and update the plot on every event with `ISISCallbacks`.