diff --git a/src/charts/shape.js b/src/charts/shape.js
index f957bcd..8422b7e 100644
--- a/src/charts/shape.js
+++ b/src/charts/shape.js
@@ -2,6 +2,7 @@
 
 import {invokeCallback, mixin} from "@/common.js";
 import {chart, createShape, deleteShapeId, dragging, draggingShapeIds, drawShape, widget} from "@/charts/chart.js";
+import {unique} from "@/misc.js";
 
 
 //
@@ -263,7 +264,8 @@ function dirtyKeys(propsA, propsB) {
         return propsA === null ? [] : [...Object.keys(propsA)]
     if (propsA===null)
         return [...Object.keys(propsB)]
-    return [...(new Set(Object.keys(propsB)).union(new Set(Object.keys(propsA))))].filter((k)=> !(k in propsA) || propsA[k] !== propsB[k])
+    const keys = unique([...Object.keys(propsA), ...Object.keys(propsB)])
+    return keys.filter((k)=> !(k in propsA) || propsA[k] !== propsB[k])
 }
 
 
diff --git a/src/misc.js b/src/misc.js
index 0131e63..b053381 100644
--- a/src/misc.js
+++ b/src/misc.js
@@ -218,3 +218,14 @@ export function lineColor(buy, index) {
     return c.string()
 }
 
+
+export function unique(arr) {
+    const u = {}, a = [];
+    for(let i = 0, l = arr.length; i < l; ++i){
+        if(!u.hasOwnProperty(arr[i])) {
+            a.push(arr[i]);
+            u[arr[i]] = 1;
+        }
+    }
+    return a;
+}