Get effective n data for the variable.

def ess(self):
        """Get effective n data for the variable."""
        _, y_vals, values, colors = self.labels_ticks_and_vals()
        for y, value, color in zip(y_vals, values, colors):
            if value.ndim != 2 or value.shape[0] < 2:
                yield y, None, color
            else:
                yield y, _get_ess(value), color