AllPlot Calibration Markers Tweak

.. only show them on power plots

.. don't show text

.. make them /really/ faint

NOTE: This is because crank based powermeters (e.g. SRM) will send
      calibration messages every time you freewheel for 3 secs or more and
      modern headunits (e.g. Nav2Coach) will record and adopt them.

      As you can guess, I have an N2C + SRMs and get > 20 or 30 calibrations
     every ride with the latest N2C firmware, so this one is for me.