Following error happened on index.php?date=2018-03-18&viewDataTable=graphVerticalBar&module=VisitTime&action=getByDayOfWeek&widget=1&columns_to_display=nb_actions%2Cnb_uniq_visitors&idSite=1&period=week

"message":"Cannot unset string offsets","file":"plugins\/CoreVisualizations\/JqplotDataGenerator.php","line":106

Haven't tried to reproduce myself so not sure if it is an easy fix or not.