code: private void drawGuideline(Point[] points, Canvas canvas) {
          Paint paint = new Paint();
          paint.setAntiAlias(true);
          paint.setColor(_guideline_color);
          paint.setStyle(Paint.Style.STROKE);
          paint.setStrokeWidth(_guideline_width);
          for (Point point : points) {
              canvas.drawLine(point.x, point.y, center.x, center.y, paint);
          }
      }
docs: 画引导线