Uses a traversed field (product__size) as the crosstab axis.
demo_app.reports.CrosstabWithTraversingField
class CrosstabWithTraversingField(CrosstabReport):
report_title = _("Cross tab Report With Traversing Field")
report_description = _("Uses a traversed field (product__size) as the crosstab axis.")
crosstab_field = "product__size"