int mypoint = 1;

void setup()
{  
  size(200,200);
  background(128);

  // speed it up
  framerate(30);
}

void draw()
{
  // redraw the background
  background(128);
  
  mypoint = mypoint + 3;  // make it move faster

  // use an IF statement to keep mypoint from going off the screen
  if (mypoint >= width) // if it is greater than OR equal to width
  {
     mypoint = 0; 
  }
  
  // draw a line from the mouse to the point
  line(mouseX,mouseY,mypoint,mypoint);  
  
  // draw a line from the 0, 0 point to the bottom right edge
  line(0,0,width,height);
}