The timechart UI expects full times, not days of week. If you're willing to lose the hourly details within a day, something like this should work:
index=volume_hourly_summary report="volumebyhour" earliest=-4w@w latest=@w | timechart span=1d sum(count) as TotalVolume | timewrap w series=exact| eval time=strftime(_time, "%A") | fields - _time | table time *
(note, this removes _time and uses 'time', and I replaced 1h with 1d)
You might have to sort the days of the week.
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