         type: 'Number'
         summary: "Skip values below that limit."
         default: 10
-      limit_days:
+      days:
         type: 'Number'
         summary: "Return data from now until this many days ago."
         default: 14

    summary: "Avarage value of book collection over time."
    description: "This will aggregate the value data of a collection of books on a per day basis. By default, this will use the current user's collection of books and summarize the value of the whole collection for the last 14 days."
    needsAuth: true
    params:
      per_book:
        type: 'Bool'

  # params
  min_value = parseInt req.param('min_value'), 10
  per_book = req.param('per_book')
  limit_days = parseInt(req.param('days'), 10) || 14

  match_users_books =
    $match:
