graph: the stack & legend sort sync was not working correctly, the z-index sorting that happened in after the legend sort order was applied and messed with the order even though the sort function returned zero for all entries, combined the sort function to one sort function, fixes #9789 (#9797)

public/app/plugins/panel/graph/graph.ts

@@ -375,20 +375,8 @@ function graphDirective($rootScope, timeSrv, popoverSrv, contextSrv) {
         var sortOrder = panel.legend.sortDesc;
         var haveSortBy = sortBy !== null || sortBy !== undefined;
         var haveSortOrder = sortOrder !== null || sortOrder !== undefined;
-
-        if (panel.stack && haveSortBy && haveSortOrder) {
-          var desc = desc = panel.legend.sortDesc === true ? -1 : 1;
-          series.sort((x, y) => {
-            if (x.stats[sortBy] > y.stats[sortBy]) {
-              return 1 * desc;
-            }
-            if (x.stats[sortBy] < y.stats[sortBy]) {
-              return -1 * desc;
-            }
-
-            return 0;
-          });
-        }
+        var shouldSortBy = panel.stack && haveSortBy && haveSortOrder;
+        var sortDesc = panel.legend.sortDesc === true ? -1 : 1;
 
         series.sort((x, y) => {
           if (x.zindex > y.zindex) {
@@ -399,6 +387,15 @@ function graphDirective($rootScope, timeSrv, popoverSrv, contextSrv) {
             return -1;
           }
 
+          if (shouldSortBy) {
+            if (x.stats[sortBy] > y.stats[sortBy]) {
+              return 1 * sortDesc;
+            }
+            if (x.stats[sortBy] < y.stats[sortBy]) {
+              return -1 * sortDesc;
+            }
+          }
+
           return 0;
         });