matplotlib颜色条缺少轴

Question

    for i in range(5000):
        weights = np.random.random(12)
        weights /= np.sum(weights)
        ER = np.sum(log_returns.mean() * weights) * 252
        SD = np.sqrt(np.dot(weights.T , np.dot(an_cov,weights)))
        rf = 0.02
        Sharpe = (ER - rf)/SD
        port_return.append(ER)
        port_vol.append(SD)
        port_sharpe.append(Sharpe)
        s_weight.append(weights)
P = {'Portfolio Expected Return':port_return, 'Portfolio Standard 
Deviation':port_vol, 'Stock Weights':s_weight , 'Portfolio Sharpe 
Ratio':port_sharpe}
Q = pd.DataFrame(P)

    plt.style.use('seaborn')
    Q.plot.scatter(x='Portfolio Standard Deviation',y='Portfolio Expected 
    Return',c = 'Portfolio Sharpe Ratio',figsize=(12,9), grid=True,  
    cmap='RdYlGn')
    plt.xlabel('Volatility (Std. Deviation)')
    plt.ylabel('Expected Returns')
    plt.title('random portfolios')
    plt.show()

我尝试多次绘制此图，但是每次X轴为何丢失？色标标签，Y轴和标题正常，但只有X轴不起作用。