请问有什么办法可以reshape 这样的wide成面板数据呢？谢谢

date	ac	bd	as
31/12/1998	0.491	0.4878	0.889
31/03/1999	0.3965	0.3587	0.6128
30/06/1999	0.4975	0.3575	0.8822

--> reshape

date	name	sale
31/12/1998	ac	0.491
31/03/1999	ac	0.3965
30/06/1999	ac	0.4975
31/12/1998	bd	0.4878
31/03/1999	bd	0.3587
30/06/1999	bd	0.3575
31/12/1998	as	0.889
31/03/1999	as	0.6128
30/06/1999	as	0.8822