Weeks in Chartio start with Monday and end on Sunday since they are defined using the ISO week date system. More information about how weeks are defined in Chartio is located here.
In order to edit your charts so they report activity from Sunday to Saturday, you can use a date range in your query using relative date variables such as:
This will subtract one day from the start and end of the current ISO week, selecting the days Sunday through Saturday. However, this approach may cause issues with data spanning multiple years.
You can see all the relative date variables here.