Adjust grid label font to zoom factor

def _zoom_labels(self, zoom):
        """Adjust grid label font to zoom factor"""

        labelfont = self.grid.GetLabelFont()
        default_fontsize = get_default_font().GetPointSize()
        labelfont.SetPointSize(max(1, int(round(default_fontsize * zoom))))
        self.grid.SetLabelFont(labelfont)