diff --git a/addons/web/static/src/js/views/graph_widget.js b/addons/web/static/src/js/views/graph_widget.js
index 8e6a574a35d..d1452fed2b1 100644
--- a/addons/web/static/src/js/views/graph_widget.js
+++ b/addons/web/static/src/js/views/graph_widget.js
@@ -66,7 +66,8 @@ return Widget.extend({
             values = [];
             if (this.groupbys.length === 1) data_pt.value = [data_pt.value];
             for (j = 0; j < data_pt.value.length; j++) {
-                values[j] = this.sanitize_value(data_pt.value[j], data_pt.grouped_on[j]);
+                var field = _.isArray(data_pt.grouped_on) ? data_pt.grouped_on[j] : data_pt.grouped_on;
+                values[j] = this.sanitize_value(data_pt.value[j], field);
             }
             value = is_count ? data_pt.length : data_pt.aggregates[this.measure];
             this.data.push({