what is a y-axis in /joint_states/velocity[5] in rqt_plot ?

edit

From the sensor_msgs/JointState comments:

The state of each joint (revolute or prismatic) is defined by:
   * the position of the joint (rad or m),
   * the velocity of the joint (rad/s or m/s) and 
   * the effort that is applied in the joint (Nm or N).

So rad/s or m/s, depending on whether the joint is a revolute or linear joint.

Note that this is what the message semantics are specced to be. If a node abuses the message and puts ticks/foot in there, that will work, but will obviously violate semantics.