feat: show parent scales on variance chart

app/models/scale.rb

@@ -90,3 +90,4 @@ class Scale < ApplicationRecord
     survey_items.student_survey_items
   end
 end
+

app/presenters/overview/overview_presenter.rb

@@ -60,3 +60,4 @@ class Overview::OverviewPresenter
     end
   end
 end
+