Account for unfinished events in log printer

Show unfinished events as being in progress when pretty-printing a log
file's contents.
Account for unfinished events in histogram

In this case, by ignoring them.
Account for unfinished events in ribbon

Show unfinished events as grayscale vertical bands along with the blue
spikes down from the top for successes and the red spikes up from the
bottom for failures.

Also initialize counts to 1 instead of 0 to avoid division-by-zero
errors if we're missing examples of one or more of the event status
cases in our input feed (an issue noticed during testing of this
change).
Account for unfinished events in rolling_stack

The rolling stack visualization is meant to show categorized failures
relative to event completion rates, so in-progress events are ignored in
this visualization.
Account for unfinished events in scatter

Show unfinished events as gray dots on the scatter visualization.