BUG: fix precision to specify decimal places
- [x] closes #13257 
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

Use 'f' as a string conversion type to specify the number of decimals to
represent. The 'g' format will instead fall back to the 'e' format
causing the unexpected behavior seen in GHI13257.

See: [old-string-formatting](https://docs.python.org/3.5/library/stdtypes.html#old-string-formatting)

Replicated code from the issue, with output:
<img width="702" alt="example output with precision" src="https://cloud.githubusercontent.com/assets/7526790/17386623/77eb83f4-59ba-11e6-8193-fde0e839a184.png">
