Account for unfinished events in scatter

Show unfinished events as gray dots on the scatter visualization.
Account for unfinished events in status_stack

Ignore in-progress events in status_stack visualization, as it is meant
to display the relative success/failure frequencies of finished events.
Account for unfinished events in sweep

Show in-progress events as grey arcs branching both up and down from the
center line an an indication of the uncertainty of their eventual
completion status (consider cats in boxes).