// A new variable, for gravity (i.e. acceleration).   
// We use a relatively small number (0.1) because this accelerations accumulates over time, increasing the speed.   
// Try changing this number to 2.0 and see what happens.
PVector   Origin, Scale;
PVector[]  wv = new PVector[3];
PVector[]  sv = new PVector[3];
PFont fontA;
float  x, y;

void setup() {
  size(600,600);
  smooth();
  noLoop();
  Origin = new PVector(150, 150);
  Scale = new PVector(50, 50); 
  for(int i=0; i<3; i++) {
    wv[i] = new PVector(0, 0);
    sv[i] = new PVector(100, 100);
  }    
  fontA = loadFont("ComicSansMS-Bold-32.vlw");
  textAlign(CENTER);
  textFont(fontA, 8);
}

PVector World2Screen(PVector v){
//  v.x=v.x*Scale.x+Origin.x;
//  v.y=height-v.y*Scale.y-Origin.y;
  
  v.mult(Scale);
  v.add(Origin);
  v.y=height-v.y;
  return v;
}

void draw() {
  background(200);
  fill(0);
  stroke(0);
  rectMode(CENTER);

  sv[0]=World2Screen(wv[0]);  
  ellipse(sv[0].x, sv[0].y, 5, 5);
  text("O", sv[0].x-5, sv[0].y+10);
  wv[0].set(-1,0,0);
  wv[1].set(5,0,0);
  sv[0]=World2Screen(wv[0]);  
  sv[1]=World2Screen(wv[1]);  
  line(sv[0].x, sv[0].y, sv[1].x, sv[1].y);  

  wv[0].set(0,-1,0);
  wv[1].set(0,5,0);
  sv[0]=World2Screen(wv[0]);  
  sv[1]=World2Screen(wv[1]);  
  line(sv[0].x, sv[0].y, sv[1].x, sv[1].y);  

  for(int i=1; i<=5; i++) {
    wv[0].set(i,0,0);
    sv[0]=World2Screen(wv[0]);  
    line(sv[0].x, sv[0].y-5, sv[0].x, sv[0].y+5);
    text(i, sv[0].x, sv[0].y+15);
  }
  
  for(x=0; x<5; x+=0.1) {
    y=ft(x);
    wv[0].set(x,y,0);
    sv[0]=World2Screen(wv[0]);  
    rect(sv[0].x, sv[0].y, 1, 1);
  }
}

float ft(float x)
{
  return x*x-x;
}