Remove the last *count* ser elements from *plotArea*. Any xChart
        elements having no ser child elements after trimming are also
        removed.

def _trim_ser_count_by(self, plotArea, count):
        """
        Remove the last *count* ser elements from *plotArea*. Any xChart
        elements having no ser child elements after trimming are also
        removed.
        """
        extra_sers = plotArea.sers[-count:]
        for ser in extra_sers:
            parent = ser.getparent()
            parent.remove(ser)
        extra_xCharts = [
            xChart for xChart in plotArea.iter_xCharts()
            if len(xChart.sers) == 0
        ]
        for xChart in extra_xCharts:
            parent = xChart.getparent()
            parent.remove(xChart)