I already tried to add _order = 'project_id asc' in the account.analytic.line model, and it does kind of sort the lines, but lines with a timesheet on Monday are sorted, and then the ones which don't have a timesheet on Monday, but one on Tuesday, and then the ones which don't have a timesheet on Monday or Tuesday, but have one Wednesday, etc.
I'd like them to be sorted globally, not separately according to the timesheet's day.