// Graph the function y=x^2 - 10, x=-20...20
// Range of function: [-10, 390]

// Let's say 400 will appear at screenY = 50
// y=0 will appear at screenY = 450
// y=-10 will appear at screenY=460
// So  screenY = 450 - y

// Let's put x=-20 at screenX = 50
// x=0 at screenX = 250
// x=20 at screenX = 450
// So  screenX = (x+20)*10 + 50


void setup() {
  float x, prevX;
  float y, prevY;

  size (500, 500);

  // Generate the first (x,y) pair and save it as prev
  prevY = (-20*-20) - 10;

  for ( x=-19 ; x<=20 ; x++ ) {
    y = x*x - 10;
    drawLine (x-1, prevY, x, y);
    prevY = y;
  }
}

void drawLine (float prevX, float prevY, float x, float y) {
  line (screenX (prevX), screenY (prevY), screenX (x), screenY (y));
}

float screenX (float x) {
  return ( (x+20)*10 + 50 );
}

float screenY (float y) {
  return ( 450-y );
}