This function cycles through numbering off, normal, and relative numbering.
" toggle number mode
function! ToggleNumberMode()
if &relativenumber == 1
set norelativenumber
set number
echo "normal numbering"
elseif &number == 1
set nonumber
echo "numbering off"
else
set relativenumber
echo "relative numbering"
endif
return
endfunc
nnoremap <f6> :call ToggleNumberMode()<cr>
vnoremap <f6> :call ToggleNumberMode()<cr>
inoremap <f6> <c-o>:call ToggleNumberMode()<cr>