数据格式如下，日期是无规律的，要做以日期为自变量，其他两个为因变量的折线图（分开做两个图），日期只需显示年份，请问该怎么做？谢谢！

Date Index(Closing)  Rt(LN)
19-Dec-90 99.98 0.043164
20-Dec-90 104.39 0.044406
21-Dec-90 109.13 0.048472
24-Dec-90 114.55 0.048562
25-Dec-90 120.25 0.040898
26-Dec-90 125.27 0.000080
27-Dec-90 125.28 0.009296
28-Dec-90 126.45 0.009132
31-Dec-90 127.61 0.009593
2-Jan-91 128.84 0.009963
3-Jan-91 130.13 0.010017
4-Jan-91 131.44 0.004706
7-Jan-91 132.06 0.004684
8-Jan-91 132.68 0.004962
9-Jan-91 133.34 0.004714
10-Jan-91 133.97 0.004692
11-Jan-91 134.60  0.000446
14-Jan-91 134.66 0.000594
15-Jan-91 134.74 -0.003718
16-Jan-91 134.24 0.000074
17-Jan-91 134.25 -0.000074
18-Jan-91 134.24 0.000000
21-Jan-91 134.24 -0.003881
22-Jan-91 133.72 -0.004197
23-Jan-91 133.16 -0.004139
24-Jan-91 132.61 -0.004232
25-Jan-91 132.05 -0.004478
28-Jan-91 131.46 -0.003963
29-Jan-91 130.94 0.000000
30-Jan-91 130.94 -0.007436
31-Jan-91 129.97 -0.003546
1-Feb-91 129.51 -0.003558
4-Feb-91 129.05 -0.003649
5-Feb-91 128.58 0.004268
6-Feb-91 129.13 0.005098
7-Feb-91 129.79 0.004536
8-Feb-91 130.38 0.004515
11-Feb-91 130.97 0.002897
12-Feb-91 131.35 0.004254
13-Feb-91 131.91 0.004689
14-Feb-91 132.53 0.004517
19-Feb-91 133.13 0.003973
20-Feb-91 133.66 0.004628
21-Feb-91 134.28 0.004384
22-Feb-91 134.87 -0.003491
25-Feb-91 134.4 -0.003503
26-Feb-91 133.93 -0.003441
27-Feb-91 133.47 -0.003452
28-Feb-91 133.01 -0.003615