Do not try to cast as number when decimals option is not defined

When using renderables labels passing a text that can be casted as a number without passing the decimals option shows 'NaN' labels (see screeshot below) :

![mathbox-pr](https://cloud.githubusercontent.com/assets/5930831/3341847/6134f078-f88c-11e3-9654-8cbe7543b0d6.png)