%D                   = integer degrees
      %<width.precision>d  = floating point degrees
      %M                   = integer minutes
      %<width.precision>m  = floating point minutes
      %S                   = integer seconds
      %<width.precision>s  = floating point seconds
      %E                   = label with E/W instead of +/-
      %N                   = label with N/S instead of +/-
For example, the command set format x "%Ddeg %5.2mmin %E" will cause
x coordinate -1.51 to be labeled as " 1deg 30.60min W".
If the xtics are left in the default state (set xtics numeric) the coordinate will be reported as a decimal number of degrees, and format will be assumed to contain normal numeric format specifiers rather than the special set above.
To output degrees/minutes/seconds in a context other than axis tics, such as
placing labels on a map, you can use the relative time format specifiers
%tH %tM %tS for strptime.  See time_specifiers (p. 
), strptime (p. 
).