You could do something like this, run over -7d@d to @d+d in my case:
| tstats prestats=t count where index=_internal by _time span=1h | timechart span=1h count
| eval wday = strftime(_time, "%a") | where wday != "Sat" AND wday != "Sun" | fields - wday
| timewrap d
The first line loads an example timechart, the second throws out Saturdays and Sundays, and finally it runs timewrap. I get a chart for latest_day, 1day_before, 2days_before, 5days_before, 6days_before, 7days_before - note how 3 and 4 days before are missing, those would be Saturday and Sunday.
