Set a label for the plot.

        :param text: the label text.
        :param location: the location of the label inside the plot.  May
            be one of 'center', 'upper right', 'lower right', 'upper
            left', 'lower left'.
        :param style: any TikZ style to style the text.

def set_label(self, text, location='upper right', style=None):
        """Set a label for the plot.

        :param text: the label text.
        :param location: the location of the label inside the plot.  May
            be one of 'center', 'upper right', 'lower right', 'upper
            left', 'lower left'.
        :param style: any TikZ style to style the text.

        """
        if location in RELATIVE_NODE_LOCATIONS:
            label = RELATIVE_NODE_LOCATIONS[location].copy()
            label['text'] = text
            label['style'] = style
            self.label = label
        else:
            raise RuntimeError('Unknown label location: %s' % location)