Trended Report Without any dimension

[sanjoyforever]

I am not sure how to create a trended report wihout any dimension. The most logical is accountsummary dimension however it is not working
I tried using this but it is returning blank dataframe.

report_def = ReportDefinition( dimensions= "accountsummary", metrics="visits", date_from="2018-11-16", date_to="2018-11-19", granularity = "day", source="datawarehouse" )