def calculate_displacement(cmx,cmy):
    """
    Calculate displacement (aka. step values) of (x,y) coordinates.

    Keyword arguments:
    cmx -- list of floating x coordinates for center of mass of 3D array
    cmy -- list of floating y coordinates for center of mass of 3D array
    """
    
    x_disp = []
    y_disp = []
    
    #  Create lists to multiply through
    x_init = cmx[:]
    x_final = cmx[1:]
    y_init = cmy[:]
    y_final = cmy[1:]
    
    #  Calculate displacement
    for (x_init_value, x_final_value) in zip(x_init, x_final):
        disp_x_value = x_final_value - x_init_value
        x_disp.append(disp_x_value)
        
    for (y_init_value, y_final_value) in zip(y_init, y_final):
        disp_y_value = y_final_value - y_init_value
        y_disp.append(disp_y_value)
        
    return x_disp, y_disp