It's for sending data to a HPGL Plotter

Not too sure to understand fully your request, but perhaps you can look at split() .

You can tweak the frame rate, and send one line per frame, for example.

Yes, I understand,  but how? I should use a counter for it, but where should I place it? I'm confused.

Quick schematic answer in pseudo-code:

String[] lines; // Fill it with the result of split()
int counter;

void setup()
{
  // Do the split here
}

void draw()
{
  sendData(lines[counter++]);
  // Do other stuff
}