require 'tk'

c = TkCanvas.new.pack
c.bind( 'B1-Motion', proc{|x, y| elongate_line( c, x, y ) }, "%x %y" )

def elongate_line( c, x, y )
  c.coords('line', 30, 30, x, y )
end

TkcLine.new(c, 30, 30, 50, 30, 'width'=>2 ).addtag('line')

Tk.mainloop