1. For 'Format' method, the format string is handled by c++, I don't suggest to format at first in go side.
2. To add a new parameter named 'args' will confuse user, unless they read the document which we don't have, then more confuse. For this kind of operation, I suggest we keep them out of library, to keep the simplicity of API.

okey, @AllenDang what about

func Labelf(format, string, args ...interface{})*Label{
     return Label(fmt.Sprintf(fomrmat,args...)
}

it wouldn't be so confusing

okey, @AllenDang what about

func Labelf(format, string, args ...interface{})*Label{
     return Label(fmt.Sprintf(fomrmat,args...)
}

it wouldn't be so confusing

This would be good! :P

@AllenDang what about (*SliderInt).Formatf is it ok?

@gucio321 For existing 'Format' method, I need to refact them to use go's format instead. Let's add format to label first. After that I will follow your pattern to deal with 'Format'.

@AllenDang the PR is ready