Plot available playlist bitrates

Add light grey dotted lines for each available video bitrate.

@@ -427,6 +427,11 @@ form label {
   stroke: #2ca02c;
   opacity: 0.25;
 }
+.line.bitrate {
+  stroke: #999;
+  opacity: 0.5;
+  stroke-dasharray: 5, 5;
+}
 
 .timeline {
   color: #888;

@@ -321,6 +321,15 @@
         .style('text-anchor', 'end')
         .text('Bitrate (kb/s)');
 
+      // playlist bitrate lines
+      svg.selectAll('.line.bitrate')
+        .data(data.options.playlists)
+      .enter().append('path')
+        .attr('class', 'line bitrate')
+        .attr('d', function(playlist) {
+          return 'M0,' + y(playlist) + 'L' + width + ',' + y(playlist);
+        });
+
       // bandwidth line
       svg.selectAll('.bandwidth').remove();
       svg.append('path')