This is a Maximum Usable Frequency Map in MHz.    Generally it tells you the highest frequency you can use for hops of 3,000 km by looking at the midway point between you and your target.  For longer hops, break your path down into 3,000 km segments, and choose the lowest frequency of the midway points.  For information about how to use this map, please visit the owners website.  http://www.spacew.com/www/realtime.php