<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def example2():
 """ Compute the GRADEV of a nonstationary white phase noise. """

    N=1000 # number of samples
    f = 1 # data samples per second
    s=1+5/N*np.arange(0,N)
    y=s*np.random.randn(1,N)[0,:]
    x = [xx for xx in np.linspace(1,len(y),len(y))]
    x_ax, y_ax, (err_l, err_h) , ns = allan.gradev(y,data_type='phase',rate=f,taus=x)
    plt.loglog(x_ax, y_ax,'b.',label="No gaps")
    y[int(0.4*N):int(0.6*N,)] = np.NaN  # Simulate missing data
    x_ax, y_ax, (err_l, err_h), ns = allan.gradev(y,data_type='phase',rate=f,taus=x)
    plt.loglog(x_ax, y_ax,'g.',label="With gaps")
    plt.grid()
    plt.legend()
    plt.xlabel('Tau / s')
    plt.ylabel('Overlapping Allan deviation')
    plt.show()