Fix const in multigraph.htm

Author: linev

demo/multigraph.htm

@@ -22,7 +22,7 @@
       function updateGUI() {
         // this is just generation of graph
          const npoints = 20;
-         const xpts = [], ypts = [];
+         let xpts = [], ypts = [];
          for (let i = 0; i < npoints; i++) {
             if (cnt % 2 === 1) {
                xpts.push(i);