Improve points visualization in metrics_plotter.

Bug: None
Change-Id: Id86c67ae9e1690817b98d8f62db0c9d05911a58a
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/231680
Reviewed-by: Artem Titov <titovartem@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#34963}

rtc_tools/metrics_plotter.py

@@ -88,7 +88,8 @@ def main():
         plt.ylabel('%s (%s)' % (metric[GRAPH_NAME], metric[UNITS]))
         plt.xlabel('time (s)')
         plt.title(metric[GRAPH_NAME])
-        plt.plot(x_values, y_values)
+        plt.plot(x_values, y_values, marker='x', markersize=3,
+                 markeredgecolor='red')
 
     plt.show()