我有一個如下所示的數據框：-df1.set_index('date', inplace=True)date        symbol   roe2019-12-31  FMC 21.21892018-12-31  FMC 16.74562017-12-31  FMC -4.91672016-12-31  FMC 6.55802015-12-31  FMC -11.14082014-12-31  FMC 19.66752013-12-31  FMC 23.63722012-12-31  FMC 29.79162011-12-31  FMC 32.22912010-12-31  FMC 18.37372009-12-31  FMC 22.68112008-12-31  FMC 35.85472007-12-31  FMC 15.62532006-12-31  FMC 14.26452005-12-31  FMC 11.29992019-12-31  VMC 21.21892018-12-31  VMC 16.74562017-12-31  VMC -4.91672016-12-31  VMC 6.55802015-12-31  VMC -11.14082014-12-31  VMC 19.66752013-12-31  VMC 23.63722012-12-31  VMC 29.79162011-12-31  VMC 32.22912010-12-31  VMC 18.37372009-12-31  VMC 22.68112008-12-31  VMC 35.85472007-12-31  VMC 15.62532006-12-31  VMC 14.26452005-12-31  VMC 11.29992019-12-31  APD 21.21892018-12-31  APD 16.74562017-12-31  APD -4.91672016-12-31  APD 6.55802015-12-31  APD -11.14082014-12-31  APD 19.66752013-12-31  APD 23.63722012-12-31  APD 29.79162011-12-31  APD 32.22912010-12-31  APD 18.37372009-12-31  APD 22.68112008-12-31  APD 35.85472007-12-31  APD 15.62532006-12-31  APD 14.26452005-12-31  APD 11.29992019-12-31  MLM 21.21892018-12-31  MLM 16.74562017-12-31  MLM -4.91672016-12-31  MLM 6.55802015-12-31  MLM -11.14082014-12-31  MLM 19.66752013-12-31  MLM 23.63722012-12-31  MLM 29.79162011-12-31  MLM 32.22912010-12-31  MLM 18.37372009-12-31  MLM 22.68112008-12-31  MLM 35.85472007-12-31  MLM 15.62532006-12-31  MLM 14.26452005-12-31  MLM 11.29992019-12-31  NEM 21.21892018-12-31  NEM 16.74562017-12-31  NEM -4.91672016-12-31  NEM 6.55802015-12-31  NEM -11.14082014-12-31  NEM 19.66752013-12-31  NEM 23.63722012-12-31  NEM 29.79162011-12-31  NEM 32.22912010-12-31  NEM 18.37372009-12-31  NEM 22.68112008-12-31  NEM 35.85472007-12-31  NEM 15.62532006-12-31  NEM 14.26452005-12-31  NEM 11.2999我已經將日期設置為索引，并使用 groupby 函數來繪制圖表，代碼如下所示：-df1.groupby('symbol')['roe'].plot(figsize = (8, 6), legend=True)輸出僅顯示 1 行。除了輸出將顯示 5 個不同的圖形之外，因為數據框中有 5 個不同的符號，我可以知道如何解決這個問題嗎？