The set dashtype command allows you to define a dash pattern that can
then be referred to by its index.  This is purely a convenience, as anywhere
that would accept the dashtype by its numerical index would also accept an
explicit dash pattern.
Example:
    set dashtype 5 (2,4,2,6)   # define or redefine dashtype number 5
    plot f1(x) dt 5            # plot using the new dashtype
    plot f1(x) dt (2,4,2,6)    # exactly the same plot as above
    set linetype 5 dt 5        # always use this dash pattern with linetype 5
    set dashtype 66 "..-"      # define a new dashtype using a string
See also dashtype (p. 
).