使用希腊符号自定义图例标签

我单击链接Legends ggplot来自定义图例标签。我以他们的示例为例，尝试应用LINE AND POINT图的情况。目前，我可以在图条的图例中添加希腊符号，但LINE AND POINT除外。

您能指出这里出了什么问题对您有帮助吗？

library(ggplot2)
# Two variables
df <- read.table(header=TRUE, text='
 cond yval
                 A 2
                 B 2.5
                 C 1.6
                 ')

# Three variables
df2 <- read.table(header=TRUE, text='
                  cond1 cond2 yval
                  A      I 2
                  A      J 2.5
                  A      K 1.6
                  B      I 2.2
                  B      J 2.4
                  B      K 1.2
                  C      I 1.7
                  C      J 2.3
                  C      K 1.9
                  ')

# BAR IS OK 
a<- ggplot(df2, aes(x=cond1, y=yval)) + 
  geom_bar(aes(fill=cond2),   # fill depends on cond2
           stat="identity",
           colour="black",    # Black outline for all
           position=position_dodge()) # Put bars side-by-side instead of stacked
a + scale_fill_discrete(name = "Test legend" , 
                            breaks=levels(df2$cond2), 
                            labels=c(expression(alpha[1]),expression(alpha[2]), expression(alpha[3]))) # add my own legend with Greek notation 


#LEGEND FOR LINE DOES NOT WORK
# Lines and points; colour depends on cond2
b<-ggplot(df2, aes(x=cond1, y=yval)) + 
  geom_line(aes(group=cond2), colour=df2$cond2) + # colour, group both depend on cond2
  geom_point(aes(colour=cond2),               # colour depends on cond2
             size=3)    
b+ + scale_linetype_discrete(name = "Test legend" , 
                         breaks=levels(df2$cond2), 
                         labels=c(expression(alpha[1]),expression(alpha[2]), expression(alpha[3]))) # add my own legend with Greek notation