Stack overlapping intervals.

        Assumes that each set has the same horizontal position

def strategy(data, params):
        """
        Stack overlapping intervals.

        Assumes that each set has the same horizontal position
        """
        vjust = params['vjust']

        y = data['y'].copy()
        y[np.isnan(y)] = 0
        heights = np.append(0, y.cumsum())

        if params['fill']:
            heights = heights / np.abs(heights[-1])

        data['ymin'] = np.min([heights[:-1], heights[1:]], axis=0)
        data['ymax'] = np.max([heights[:-1], heights[1:]], axis=0)
        # less intuitive than (ymin + vjust(ymax-ymin)), but
        # this way avoids subtracting numbers of potentially
        # similar precision
        data['y'] = ((1-vjust)*data['ymin'] + vjust*data['ymax'])
        return data