Pass label and plot style to the TimeSeries' constructor.

BUG=webrtc:7323

Review-Url: https://codereview.webrtc.org/2750583002
Cr-Commit-Position: refs/heads/master@{#17237}
diff --git a/webrtc/tools/event_log_visualizer/plot_base.cc b/webrtc/tools/event_log_visualizer/plot_base.cc
index f27fcd6..baee3df 100644
--- a/webrtc/tools/event_log_visualizer/plot_base.cc
+++ b/webrtc/tools/event_log_visualizer/plot_base.cc
@@ -71,5 +71,15 @@
   title_ = title;
 }
 
+TimeSeries* Plot::AddTimeSeries(const char* label, PlotStyle style) {
+  series_list_.emplace_back(label, style);
+  return &series_list_.back();
+}
+
+TimeSeries* Plot::AddTimeSeries(const std::string& label, PlotStyle style) {
+  series_list_.emplace_back(label, style);
+  return &series_list_.back();
+}
+
 }  // namespace plotting
 }  // namespace webrtc