How to add a trigger to an oscilloscope program?

I'm working on a very bare bones oscilloscope program using the minim library:

import ddf.minim.*;

Minim minim;
AudioInput in;
color white;

void setup()
{
  size(512, 100, P2D);
  white = color(255);
  colorMode(HSB,100);
  minim = new Minim(this);
  minim.debugOn();

  in = minim.getLineIn(Minim.STEREO, 512);
  background(0);
}

void draw()
{
  background(0);
  // draw the waveforms
  for(int i = 0; i < in.bufferSize() - 1; i++)
  {
    stroke((1+in.left.get(i))*50,100,100);
    line(i, 50 + in.left.get(i)*50, i, 50 + in.left.get(i+1)*50);
  }
}


void stop()
{
  in.close();
  minim.stop();
  super.stop();
}

My question is, how do I make the waveform trigger/stabilize like an actual oscilloscope? I'm trying to figure that out so I can make the waveform stay at a certain spot so it doesn't jump around. Basically keep the waveform stable at all times.

Thanks for your help.