面板数据结构（含趋势图）

def solve():
    import pandas as pd
    import matplotlib.pyplot as plt
    
    df = pd.read_csv('https://liangdaima.com/static/data/wooldridge/wagepan.csv')
    
    plt.figure(figsize=(12,6))
    for nr in df['nr'].unique()[:5]:
        individual = df[df['nr'] == nr]
        plt.plot(individual['year'], individual['lwage'], label=f'个体{nr}')
    
    plt.title('前5个个体工资趋势', fontsize=14)
    plt.xlabel('年份', fontsize=12)
    plt.ylabel('对数工资', fontsize=12)
    plt.legend()
    plt.grid(alpha=0.3)
    plt.tight_layout()
    plt.show()
    
    print(df['year'].nunique())