def get_x( vx, vy, x1, y1, y_ref ):
    """
    Helper function for draw_lines
    Calculates 'x' matching: 2 points on a line, its slope, and a given 'y' coordinate.
    """
    m = vy / vx
    b = y1 - ( m * x1 )
    x = ( y_ref - b ) / m
    return x