条形图：Titanic各舱级生还率

def solve():
    import pandas as pd
    import matplotlib.pyplot as plt
    url = '____'
    df = pd.read_csv(____)
    sr = df.groupby(____)['____'].mean().round(____)
    print(____)
    sr.plot(____)
    plt.xlabel(____); plt.ylabel(____); plt.title(____)
    plt.xticks(____); plt.tight_layout(); plt.show()