Allow for custom number formatting function in barchart

[bcicen]

Allows a custom function to be defined when creating bar labels, useful in cases where the underlying "real" value might want to be truncated, converted to human-readable byte quantities, etc.

[vielmetti]

Can we get this merged in, if it's ready? It seems to be cropping up with ctop problems.

[kenan-rhoton]

Is there any reason for this to not be merged in already?

[cjbassi]

Thanks!