Turns out we were updating the wrong ``Viz``/``DisplayState`` inside the
closure style `increment_history_view()`` (probably due to looping
through the flumes and dynamically closing in that task-func).. Instead
define the history incrementer at module level and pass in the
`DisplayState` explicitly. Further rework the `DisplayState` attrs to be
more focused around the `Viz` associated with the fast and slow chart
and be sure to adjust output from each `Viz.incr_info()` call to latest
update. Oh, and just tweaked the line palette for the moment.
FYI "treading" here is referring to the x-shifting of the curve when
the last datum is in view such that on new sampled appends the "last"
datum is kept in the same x-location in UI terms.
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