Improve plotting script

Also, add a bunch of plotting configs for the new signals that I've
added.

Change-Id: I7aa5221939351ede989b897db561b6a57f023910
diff --git a/frc971/analysis/plot_config.proto b/frc971/analysis/plot_config.proto
index c4c61c2..cd01f73 100644
--- a/frc971/analysis/plot_config.proto
+++ b/frc971/analysis/plot_config.proto
@@ -45,6 +45,9 @@
 message Axes {
   repeated Line line = 1;
   optional string ylabel = 2;
+  optional string xlabel = 3 [default = "Monotonic Time (sec)"];
+  // Whether to share an x-axis with all the other axes.
+  optional bool share_x_axis = 4 [default = true];
 }
 
 // Message representing a single pyplot figure.