Fix diagonals grid not displaying properly in portrait orientation.

                canvas.drawLine(0.0f, 0.0f, canvas.getHeight() - 1.0f, canvas.getHeight() - 1.0f, p);

                canvas.drawLine(canvas.getHeight() - 1.0f, 0.0f, 0.0f, canvas.getHeight() - 1.0f, p);

                int diff = canvas.getWidth() - canvas.getHeight();

                if (diff > 0) {

                // n.b., diff is -ve in portrait orientation

                canvas.drawLine(diff, 0.0f, diff + canvas.getHeight() - 1.0f, canvas.getHeight() - 1.0f, p);

                canvas.drawLine(diff + canvas.getHeight() - 1.0f, 0.0f, diff, canvas.getHeight() - 1.0f, p);

                }

                break;

        }

    }