public function test_draw_rectangle()
 {
     $c = new canvas(20, 4);
     //draw a point
     $this->assertTrue($c->draw_rectangle(array(1, 1), array(1, 1)));
     //draw a line
     $this->assertTrue($c->draw_rectangle(array(1, 2), array(6, 2)));
     //draw a rectangle
     $this->assertTrue($c->draw_rectangle(array(16, 1), array(20, 3)));
 }