Return figure in plot_d.
Set grid on in plots.
Add methods for yy plots and forward speed field.

Add field forward_speed which is -WHEEL_RADIUS * rear_wheel_rate. Add
method for plotting data with two separate y axes. Add methods for
common plots.
Record estimation and control thresholds.

Add new optional message type Threshold to sample.proto. New Threshold
records estimation and control thresholds for each loop.
Add thresholds to forward speed plot.
Merge remote-tracking branch 'oliverlee/gainschd' into gainschd