Revert "fix useless resynchronisation when range isn’t used": this

is most likely (part of) the #1011 cause

This reverts commit 3034672.

src/extras/synchronizer.js

@@ -158,11 +158,8 @@ var synchronize = function(/* dygraphs..., opts */) {
   };
 };
 
-function arraysAreEqualOrBothNULL(a, b) {
-  if (a === null && b === null)
-    return true;
-  if (!Array.isArray(a) || !Array.isArray(b))
-    return false;
+function arraysAreEqual(a, b) {
+  if (!Array.isArray(a) || !Array.isArray(b)) return false;
   var i = a.length;
   if (i !== b.length) return false;
   while (i--) {
@@ -180,9 +177,9 @@ function attachZoomHandlers(gs, syncOpts, prevCallbacks) {
         if (block || initial) return;
         block = true;
         var opts = {
-          valueRange: syncOpts.range ? me.yAxisRange() : null,
           dateWindow: me.xAxisRange()
         };
+        if (syncOpts.range) opts.valueRange = me.yAxisRange();
 
         for (var j = 0; j < gs.length; j++) {
           if (gs[j] == me) {
@@ -193,8 +190,8 @@ function attachZoomHandlers(gs, syncOpts, prevCallbacks) {
           }
 
           // Only redraw if there are new options
-          if (arraysAreEqualOrBothNULL(opts.dateWindow, gs[j].getOption('dateWindow')) &&
-              arraysAreEqualOrBothNULL(opts.valueRange, gs[j].getOption('valueRange'))) {
+          if (arraysAreEqual(opts.dateWindow, gs[j].getOption('dateWindow')) &&
+              arraysAreEqual(opts.valueRange, gs[j].getOption('valueRange'))) {
             continue;
           }