给定以下数据：

data <- data.frame(Train = rep(c(20,30,40,50),2), xi = runif(8,0,1), category = c(rep("0.3_val",4),rep("0.3_test",4)))
data <- rbind(data, data.frame(Train = rep(c(20,30,40,50),2), xi = runif(8,0,1), category = c(rep("0.6_val",4),rep("0.6_test",4))))

如果考虑上面代码中的数据，线型和颜色的图例将合并为一个图例：

p <- ggplot(data = data, aes(x = Train, y = xi, group = category, color = category)) + 
  geom_line(aes(color = category,linetype=category)) +  
  geom_hline(yintercept = 1) +
  theme_bw() +
  xlab(expression(italic("n"))) + 
  ylab(expression(xi)) + 
  theme(panel.border = element_blank(), panel.grid.major = element_blank(),                                                                                                                                                      panel.grid.minor = element_blank(), axis.line = element_line(colour = "black"), legend.title=element_text(size=4)) +
  theme(legend.key.size = unit(0.2, 'cm')) + guides(fill=guide_legend(title=ylab)) + 
  theme(strip.background =element_rect(color = "black",fill="white")) +
  scale_colour_manual(name="Legend", values=c("black","black","grey","grey"))+
  scale_linetype_manual(name="Legend", values=c("dashed", "twodash","dashed", "twodash","dashed", "twodash"))

print(p)

如果现在图例标签更改为希腊符号（表达式），则图例将一分为二，尽管scale_x_manual（）中的名称和标签相同：

p <- ggplot(data = data, aes(x = Train, y = xi, group = category, color = category)) + 
  geom_line(aes(color = category,linetype=category)) +  
  geom_hline(yintercept = 1) +
  theme_bw() +
  xlab(expression(italic("n"))) + 
  ylab(expression(xi)) + 
  theme(panel.border = element_blank(), panel.grid.major = element_blank(),                                                                                                                                                      panel.grid.minor = element_blank(), axis.line = element_line(colour = "black"), legend.title=element_text(size=4)) +
  theme(legend.key.size = unit(0.2, 'cm')) + guides(fill=guide_legend(title=ylab)) + 
  theme(strip.background =element_rect(color = "black",fill="white")) +
  scale_colour_manual(name="Legend", values=c("black","black","grey","grey"),
                      labels=c("0.3_val"=expression(xi[0.3]^{val}),
                               "0.3_test"=expression(xi[0.3]^{test}),
                               "0.6_val"=expression(xi[0.6]^{val}),
                               "0.6_test"=expression(xi[0.6]^{test})))+
  scale_linetype_manual(name="Legend", values=c("dashed", "twodash","dashed", "twodash"),
                        labels=c("0.3_val"=expression(xi[0.3]^{val}),
                                 "0.3_test"=expression(xi[0.3]^{test}),
                                 "0.6_val"=expression(xi[0.6]^{val}),
                                 "0.6_test"=expression(xi[0.6]^{test})))

print(p)

在scale_x_manual（）中使用表达式时，如何避免这个问题，使图例从两个变为一个？