Get data for each treeplot for the variable.

def treeplot(self, qlist, credible_interval):
        """Get data for each treeplot for the variable."""
        for y, _, label, values, color in self.iterator():
            ntiles = np.percentile(values.flatten(), qlist)
            ntiles[0], ntiles[-1] = hpd(values.flatten(), credible_interval)
            yield y, label, ntiles, color