G-code firmware for handheld plotter (Arduino) I'd like to build a plotter to work without a PC (power supply by battery).
Plotter will write only several numbers on a card (really short G-code).

Do you know any firmware, which can be easily modified for this reason?
Or do you know G-code library which could be easily implemented into my stepper controller in Arduino?

My plan is to use Repetier (printing from SD), but I prefer to print without SD.
Repetier is also too big, there are a lot of functions that I don't need.
Your question may be useful to people interested in 3d printing (the purpose of this SE Site) so, even though you're not asking a 3d printing question per se, I think this answer will be helpful...

GRBL:

GRBL is a well known and mature g-code interpreter that will run on an Arduino Uno and is free and open source.  Check it out here.

I hope this helps!  :-)