total % for an assignment group

[n_k_wilkin]

Hello
I am creating assignments via canvasapi with the assignment groups weighted. It seems that the % of total box does not show until I set it the first time within canvas manually, and then it is visible for any new assignment groups. Behind the scenes, the setting has been received. E.g. for 

a1_group = course.create_assignment_group(name='A1', group_weight=20)

canvas has registered the weighting of the group as 20%. 

Does anyone know how to get this setting switched on via the API?
thank you!
Nicola