Sets the horizontal axis for this chart.
        
        :param      axis | <XChartAxis>

def setHorizontalAxis(self, axis):
        """
        Sets the horizontal axis for this chart.
        
        :param      axis | <XChartAxis>
        """
        self._horizontalAxis = axis
        if axis:
            axis.setOrientation(Qt.Horizontal)
            self.uiXAxisVIEW.setFixedHeight(axis.minimumLabelHeight() + 5)
        
        self.uiXAxisVIEW.setVisible(axis is not None)