'Add a callback that will be called on each bar.
:param label: A name for the series values.
:type label: string.
:param callback: A function that receives a :class:`pyalgotrade.bar.Bars` instance as a parameter and returns a number or None.'

def addCallback(self, label, callback, defaultClass=LineMarker):

    self.__callbacks[callback] = self.getSeries(label, defaultClass)
