[FIX] web: Graph: group by two fields on line chart

Before this commit, when we applied two or more groupbys in line
graph, the groups displayed on the X axis where shifted to the left,
and the first one wasn't displayed at all.

related task: #1848289
Closes: #25037

addons/web/static/src/js/views/graph/graph_renderer.js

@@ -313,7 +313,7 @@ return AbstractRenderer.extend({
                     };
                 }
                 data_dict[serie].values.push({
-                    x: tick, y: this.state.data[i].value,
+                    x: tick - 1, y: this.state.data[i].value,
                 });
                 data = _.map(data_dict, identity);
             }

addons/web/static/tests/views/graph_tests.js

@@ -154,6 +154,29 @@ QUnit.module('Views', {
         });
     });
 
+    QUnit.test('displaying line chart data with multiple groupbys', function (assert) {
+        // this test makes sure the line chart shows all data labels (X axis) when
+        // it is grouped by several fields
+        assert.expect(3);
+
+        var graph = createView({
+            View: GraphView,
+            model: 'foo',
+            data: this.data,
+            arch: '<graph type="line"><field name="foo" /></graph>',
+            groupBy: ['product_id', 'bar'],
+        });
+
+        assert.strictEqual(graph.$('.nv-x text:contains(xphone)').length, 1,
+            "should contain a text element with product xphone on X axis");
+        assert.strictEqual(graph.$('.nv-x text:contains(xpad)').length, 1,
+            "should contain a text element with product xpad on X axis");
+        assert.strictEqual(graph.$('text:contains(true)').length, 1,
+            "should have an entry for each value of field 'bar' in the legend");
+
+        graph.destroy();
+    });
+
     QUnit.test('switching measures', function (assert) {
         var done = assert.async();
         assert.expect(4);