Don't pass hardcoded values

When I was trying to override the default value of since using 
def self.graph_data since=30.days.ago
    [
      {
          name: 'Admin Users',
          pointInterval: point_interval = 1.day * 1000,
          pointStart: start_point = since.to_i * 1000,
          data: self.where(type: 'Admin').delta_records_since(since)
      },
      {
          name: 'Standard Users',
          pointInterval: point_interval,
          pointStart: start_point,
          data: self.where(type: nil).delta_records_since(since)
      }
    ]
  end
The value is hardcoded as 100 in _chart.html.erb. Removing the hardcoded value from the view, the new default value would be 30.days.ago instead of 100.days.ago