fix: a memory leak appearing when some timeSeries.disabled === true

Introduced in 86574e

smoothie.js

@@ -910,18 +910,18 @@
 
     // For each data set...
     for (var d = 0; d < this.seriesSet.length; d++) {
-      context.save();
       var timeSeries = this.seriesSet[d].timeSeries;
+
+      // Delete old data that's moved off the left of the chart.
+      timeSeries.dropOldData(oldestValidTime, chartOptions.maxDataSetLength);
       if (timeSeries.disabled) {
           continue;
       }
+      context.save();
 
       var dataSet = timeSeries.data,
           seriesOptions = this.seriesSet[d].options;
 
-      // Delete old data that's moved off the left of the chart.
-      timeSeries.dropOldData(oldestValidTime, chartOptions.maxDataSetLength);
-
       // Set style for this dataSet.
       context.lineWidth = seriesOptions.lineWidth;
       context.strokeStyle = seriesOptions.strokeStyle;