Path.pde

class Path{
  Path followingPath;
  PVector start; 
  PVector end;
  Float startAngle; 
  Float endAngle;
  Path(PVector start, PVector end, Float startAngle, Float endAngle){
    this.start = start;
    this.end   = end;
    
    this.startAngle = startAngle;
    this.endAngle   = endAngle;
  }
  public void draw(){
    stroke(255);
    
    //Line between points
    
    
    float mStart = tan(startAngle);
    float mEnd   = tan(endAngle);
    
    // y = m * x + b
    // b = y - m * x
    
    float bStart = start.y - mStart * start.x;
    float bEnd   = end  .y - mEnd   * end  .x;
    
    float x = (bEnd -bStart) / (mStart-mEnd);
    float y = bStart + mStart*x;
    
    stroke(255,0,0);
    
    point(x,bStart + mStart*x);
    noFill();
    bezier(
      start.x, start.y, 
      x, y, x, y, 
      end.x, end.y
    );
    if(followingPath != null){
      followingPath.draw();
    }
  }
  
  public void addToEnd(float x, float y){
    end.add(x,y);
  }
  
  public void setEnd(float x, float y){
    end.set(x,y);
  }
  
  public void addToEndRotation(float rot){
    endAngle += rot;
    println(endAngle);
  }
  
  public Path createChild(){
    followingPath = new Path(end, end.copy(), endAngle, endAngle + 1);
    return followingPath;
  }
}

Pathbuilder.pde

Path path;
Path selected;

void setup(){
  size(400,400);
  path = new Path(
    new PVector(100,100),
    new PVector(300,300),
    radians(30),
    radians(250)
  );
  selected = path;
}

void draw(){
  background(0);
  if(!mousePressed && (mouseX + mouseY) < (width + height))
    selected.setEnd(mouseX, mouseY);
  path.draw();
}

void mouseDragged(){
  selected.addToEndRotation(radians(pmouseX - mouseX + pmouseY - mouseY));
}

void mouseClicked(){
  selected = selected.createChild();
}