diff --git a/smoothie.js b/smoothie.js
index b54c436..758414d 100644
--- a/smoothie.js
+++ b/smoothie.js
@@ -210,7 +210,7 @@
    */
   TimeSeries.prototype.append = function(timestamp, value, sumRepeatedTimeStampValues) {
 	// Reject NaN
-	if (isNaN(timestamp) || isNaN(value)){
+	if (isNaN(timestamp) || (typeof value != string && isNaN(value))){
 		return
 	}  
     // Rewind until we hit an older timestamp
@@ -339,7 +339,8 @@
   /** Formats the HTML string content of the tooltip. */
   SmoothieChart.tooltipFormatter = function (timestamp, data) {
       var timestampFormatter = this.options.timestampFormatter || SmoothieChart.timeFormatter,
-          lines = [timestampFormatter(new Date(timestamp))];
+          lines = [timestampFormatter(new Date(timestamp))],
+          label;
 
       for (var i = 0; i < data.length; ++i) {
         label = data[i].series.options.tooltipLabel || ''