re-add parseFloat to default y-axis formatters

Upgraded from 1.18 to 1.23, starting seeing undefined errors in yMinFormatter:
https://github.com/joewalnes/smoothie/blob/master/smoothie.js#L261

Noticed calls to parseFloat were removed in commit fe2d5f99bb4bcbbc877687436a0c86c3ca466a05, prior to 1.23 release.