dygogram

from The Century Dictionary.

• noun A diagram that shows both the horizontal directive magnetic force of the compass-needle on board ship under local influences and its deviation from the meridian for any latitude and sailing-course; specifically, the dygogram of Captain Colongue (1870) and of A. Smith, from which Paugger obtained the idea of his mechanical dromoscope of 1872.
• noun A diagram containing a curve generated by the motion of a line drawn from a fixed origin, and representing in direction and magnitude the horizontal component of the force of magnetism on a ship's compass-needle while the ship makes a complete circuit.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun nautical A chart showing the horizontal variation in the magnetic force acting on a ship's compass needle by the iron within the ship, as a function of course steered