生育率时序置信区间（高级图表）

def solve():
    import pandas as pd
    import matplotlib.pyplot as plt
    import numpy as np
    
    url = 'https://liangdaima.com/static/data/wooldridge/fertil1.csv'
    df = pd.read_csv(url)
    
    year_stats = df.groupby('year')['kids'].agg(['mean', 'std']).reset_index()
    plt.figure(figsize=(12,6))
    plt.fill_between(year_stats['year'], year_stats['mean'] - year_stats['std'], 
                    year_stats['mean'] + year_stats['std'], alpha=0.3, color='teal')
    plt.plot(year_stats['year'], year_stats['mean'], 'o-', color='teal', linewidth=2, markersize=8)
    plt.title('生育率时序及变异区间', fontsize=14)
    plt.xlabel('年份', fontsize=12)
    plt.ylabel('平均生育数量', fontsize=12)
    plt.grid(alpha=0.3)
    plt.tight_layout()
    plt.show()
    return "可视化代码执行完成"