Isogonic lines, or lines of equal magnetic declination, are useful as supplementary cartographic data on navigational charts. Prior to ArcGIS 10.1, a geoprocessing tool called Magnetic Isolines, available with Esri Production Mapping, was used to create isomagnetic lines based on the World Magnetic Model for dates between 1/1/2005 and 12/31/2014. With the release of 10.1, Make Isogonic Lines, a new geoprocessing model available in the Production Mapping gallery replaces the Magnetic Isolines geoprocessing tool. The model extends the Magnetic Calculator geoprocessing tool and passes the results through a suite of additional geoprocessing tools to build the isogonic lines as you see in the image below.
The World Magnetic Model is used to determine the magnetic declination at different points on the planet. Now that the tool is available as a geoprocessing model, it is easily customizable. You can download the model and use it as-is to derive the isogonic lines. You can also edit the model to use magnetic inclination instead of declination to create isolines based on the inclination value.