dygraphs_performance_update.diff

diff --git a/dygraph.js b/dygraph.js
index 3d3326e..374c355 100644
--- a/dygraph.js
+++ b/dygraph.js
@@ -1521,8 +1521,18 @@ Dygraph.prototype.drawGraph_ = function(data) {
   for (var i = 1; i < data[0].length; i++) {
     if (!this.visibility()[i - 1]) continue;
 
+    // Reduce the resolution of the dataset if necessary.
+    var data_length = data.length
+    if (this.dateWindow_) {
+      data_length = this.dateWindow_[1] - this.dateWindow_[0];
+    }
+    var step = 1;
+    if (data_length > Math.round(this.width_/4)) {
+      step = Math.round(data_length/(this.width_/4));
+    }
+
     var series = [];
-    for (var j = 0; j < data.length; j++) {
+    for (var j = 0; j < data.length; j+=step) {
       if (data[j][i] || !connectSeparatedPoints) {
         var date = data[j][0];
         series.push([date, data[j][i]]);