function! s:Gm(isVisual)
  normal! ^
  let first_col = virtcol('.')
  normal! g_
  let last_col = virtcol('.')
  execute 'normal! ' . (a:isVisual ? 'gv': '') . (first_col + last_col) / 2 . '|'
endfunction
nnoremap <silent> gm :call <SID>Gm(0)<CR>
vnoremap <silent> gm :call <SID>Gm(1)<CR>