def calculate_derivative(x_vector, y_vector):
    """
    calculate_derivative(x_vector, y_vector)
        determines the derivative of y (values) with respect to x (time)

        Parameters
        ----------
            x_vector : array_like
                    list of values (organoid size)
            y_vector : array_like
                    list of values (time)
            
        Returns
        -------
            derivative : array_like
                    list of values with the gradient at each x (time)    
    """
    derivative = []
    for i in range(1, len(x_vector)):
        delta_x = x_vector[i]-x_vector[i-1]
        delta_y = y_vector[i]-y_vector[i-1]
        v_derivative = delta_y/delta_x
        derivative.append(v_derivative)
    return derivative