> x=data.frame(num,x1,x2,x3)
> x
num x1 x2 x3
1 201412001 89 83 74
2 201412002 85 66 77
3 201412003 93 81 86
4 201412004 87 90 85
5 201412005 87 77 95
6 201412006 95 76 89
7 201412007 84 87 96
8 201412008 98 84 92
9 201412009 83 83 82
10 201412010 89 69 94
11 201412011 84 82 96
12 201412012 95 82 100
13 201412013 97 81 84
14 201412014 90 89 88
15 201412015 91 83 91
16 201412016 81 75 86
17 201412017 81 88 81
18 201412018 81 79 81
19 201412019 98 87 87
20 201412020 81 89 76
请问如何在一张图上画出三条线,都是以num为横坐标,分别以x1,x2,x3的值为纵坐标?