def __init__(self, peak_finder = None, peak_fitter = None, properties = None, **kwds):
        """
        peak_finder - A PeakFinder object.
        peak_fitter - A PeakFitter object.
        properties - Which properties to return, e.g. "x", "y", etc.
        """
        super(PeakFinderFitter, self).__init__(**kwds)

        self.peak_finder = peak_finder
        self.peak_fitter = peak_fitter
        self.properties = properties

        # The properties must include at least 'x' and 'y'.
        assert ("x" in self.properties), "'x' is a required property."
        assert ("y" in self.properties), "'y' is a required property."