Return the Y value for data point *idx* in this cache, or None if no
        value is present for that data point.

def pt_v(self, idx):
        """
        Return the Y value for data point *idx* in this cache, or None if no
        value is present for that data point.
        """
        results = self.xpath('.//c:pt[@idx=%d]' % idx)
        return results[0].value if results else None