包含两个不同分组的GGPLOT返回R中的空图

Question

这是我的示例数据框。
我将显示具有不同颜色和不同线类型的不同颜色的不同GRP s。并将这两个作为传奇。

library(dplyr)
library(ggplot2)
seed(123)
df <- data.frame(date = rep(c("Jan-2010", "Feb-2010", "Mar-2010", "Apr-2010" ,"Dec-2009"),each=5),
                 grp = c(LETTERS[1:5], LETTERS[2:6], LETTERS[1:5],LETTERS[3:7], LETTERS[1:5]),
                 subgrp = c("x","x","x","y","y","x","x","y","y","y","x","x","x","y","y",
                            "x","y","y","y","y","x","x","x","y","y"),
                 value = sample(4:10, 25, replace=TRUE))


df
       date grp subgrp value
1  Jan-2010   A      x     7
2  Jan-2010   B      x    10
3  Jan-2010   C      x     6
4  Jan-2010   D      y     4
5  Jan-2010   E      y     5
6  Feb-2010   B      x     7
7  Feb-2010   C      x     5
8  Feb-2010   D      y     4
9  Feb-2010   E      y     6
10 Feb-2010   F      y     7
11 Mar-2010   A      x     8
12 Mar-2010   B      x    10
13 Mar-2010   C      x     7
14 Mar-2010   D      y    10
15 Mar-2010   E      y     5
16 Apr-2010   C      x     4
17 Apr-2010   D      y     9
18 Apr-2010   E      y     7
19 Apr-2010   F      y     5
20 Apr-2010   G      y     5
21 Dec-2009   A      x     6
22 Dec-2009   B      x     8
23 Dec-2009   C      x     6
24 Dec-2009   D      y     8
25 Dec-2009   E      y     9

但是即使为了分组，它也会返回一个空图。

df %>%

  ggplot() +
  geom_line(aes(x=date,
                y=value,
                color=grp))

我感谢任何提示或帮助解决此问题。