drawDiamond function
Draw the Diamond shape marker
Implementation
void drawDiamond(Path path, double x, double y, double width, double height) {
path.moveTo(x - width / 2, y);
path.lineTo(x, y + height / 2);
path.lineTo(x + width / 2, y);
path.lineTo(x, y - height / 2);
path.lineTo(x - width / 2, y);
path.close();
}