sub marker{
    my($id, $mouseX, $mouseY) = @_;

	# get offset of window to underlyling canvas
	my $offsetX = $window->canvasx(0);
	my $offsetY = $window->canvasy(0);

    my $color = '#800000';
    my @items = $id->find('withtag', 'mark');

    # if marker exists, move it to new position
    if ($items[0]){
        my $moveX = $mouseX - ($markX - $offsetX);
        my $moveY = $mouseY - ($markY - $offsetY);
        for (@items){$id->move($_, $moveX, $moveY)}

    # or else create marker
    }else{
        $id->create(('line', $mouseX, $mouseY-20, $mouseX, $mouseY+20),
            -fill => $color, -tags => ['mark'],);
        $id->create(('line', $mouseX-20, $mouseY, $mouseX+20, $mouseY),
            -fill => $color, -tags => ['mark'],);
    }

	# calculate new absolute position of marker
	$markX = $mouseX + $offsetX;
	$markY = $mouseY + $offsetY;
}