Add derived signals to plotting tool

This sets things up to make it easier to plot various types of derived
signals--e.g., basic unit conversions or transformations.

Change-Id: I8b824522a679217812489b4ac31c166ee5b8db61
diff --git a/frc971/analysis/plot_configs/gyro.pb b/frc971/analysis/plot_configs/gyro.pb
index e398a2e..f246835 100644
--- a/frc971/analysis/plot_configs/gyro.pb
+++ b/frc971/analysis/plot_configs/gyro.pb
@@ -22,6 +22,10 @@
   }
   axes {
     signal {
+      channel: "CalcIMU"
+      field: "total_acceleration"
+    }
+    signal {
       channel: "IMU"
       field: "accelerometer_x"
     }