diff --git a/js/lib/d3.tail.js b/js/lib/d3.tail.js
index 0667227ee..4afaf97ff 100644
--- a/js/lib/d3.tail.js
+++ b/js/lib/d3.tail.js
@@ -32,7 +32,7 @@ d3.tail = function() {
         function mousemove() {
             if (text === false) return;
             var xoffset = ((d3.event.x + tooltip_size[0] + xmargin) > selection_size[0]) ?
-                -tooltip_size[0] - xmargin : xoffset = xmargin;
+                -tooltip_size[0] - xmargin : xmargin;
             container.style(transformProp, 'translate(' +
                 (~~d3.event.x + xoffset) + 'px,' +
                 ~~d3.event.y + 'px)');