Newline after every character

I was printing some results for a chart while, after 10 minutes of executing the test, I realised I forgot to put the \n command after the number I wanted to print. I know the numbers I was printing were smaller than 10, so I came up with a useful command to append a new line after each character in a file:

sed 's/\(.\)/\1\n/g' -i filename

And that's it. As easy as that. I'm sorry for the lack of updates but lately I've been very busy.