Iterate over models and chains for each variable.

def iterator(self):
        """Iterate over models and chains for each variable."""
        if self.combined:
            grouped_data = [[(0, datum)] for datum in self.data]
            skip_dims = {"chain"}
        else:
            grouped_data = [datum.groupby("chain") for datum in self.data]
            skip_dims = set()

        label_dict = OrderedDict()
        for name, grouped_datum in zip(self.model_names, grouped_data):
            for _, sub_data in grouped_datum:
                datum_iter = xarray_var_iter(
                    sub_data,
                    var_names=[self.var_name],
                    skip_dims=skip_dims,
                    reverse_selections=True,
                )
                for _, selection, values in datum_iter:
                    label = make_label(self.var_name, selection, position="beside")
                    if label not in label_dict:
                        label_dict[label] = OrderedDict()
                    if name not in label_dict[label]:
                        label_dict[label][name] = []
                    label_dict[label][name].append(values)

        y = self.y_start
        for label, model_data in label_dict.items():
            for model_name, value_list in model_data.items():
                if model_name:
                    row_label = "{}: {}".format(model_name, label)
                else:
                    row_label = label
                for values in value_list:
                    yield y, row_label, label, values, self.model_color[model_name]
                    y += self.chain_offset
                y += self.var_offset
            y += self.group_offset