def _tooltip_data(self, node, value, x, y, classes=None, xlabel=None):
        """Insert in desc tags informations for the javascript tooltip"""
        self.svg.node(node, 'desc', class_="value").text = value
        if classes is None:
            classes = []
            if x > self.view.width / 2:
                classes.append('left')
            if y > self.view.height / 2:
                classes.append('top')
            classes = ' '.join(classes)

        self.svg.node(node, 'desc', class_="x " + classes).text = to_str(x)
        self.svg.node(node, 'desc', class_="y " + classes).text = to_str(y)
        if xlabel:
            self.svg.node(node, 'desc', class_="x_label").text = to_str(xlabel)
Insert in desc tags informations for the javascript tooltip