2.py

class HireVolumeChart(BaseChart):
    title = _('Hire Volume')
    template = 'admin_user_stats/modules/chart.html'
    chart_size = "580x200"
    values_count = 3 #use either days or values_count
    interval = 'days'
    queryset = BookingStockItem.objects.all()
    date_field = 'booking__estimated_dispatch'
    aggregate = Sum('quantity')