绘制各贷款等级违约率条形图

def solve():
    from pyodide.http import open_url
    from io import StringIO
    loans_raw_csv = open_url("https://data.zuihe.com/dbd/riskctrl/state_01/loans_raw.csv").read()
    import pandas as pd
    import matplotlib
    matplotlib.use('Agg')
    import matplotlib.pyplot as plt
    from io import StringIO
    df = pd.read_csv(StringIO(loans_raw_csv))
    grp = df.groupby('grade')['isDefault'].mean().reset_index()
    grp.columns=['grade','default_rate']
    grp['default_rate'] = grp['default_rate'].round(4)
    print(grp.to_string(index=False))
    fig,ax = plt.subplots(figsize=(8,4))
    ax.bar(grp['grade'],grp['default_rate'],color='steelblue')
    ax.set_xlabel('Grade'); ax.set_ylabel('违约率'); ax.set_title('贷款等级违约率')
    plt.tight_layout(); plt.show()