如何在ggplot2中为图例键混合不同的符号？

我有下面的代码生成该图。

cols <- brewer.pal(n = 3, name = 'Dark2')

p4 <- ggplot(all.m, aes(x=xval, y=yval, colour = Approach, ymax = 0.95)) + theme_bw() + 
  geom_errorbar(aes(ymin= yval - se, ymax = yval + se), width=5, position=pd) + 
  geom_line(position=pd) + geom_point(position=pd) + 
  geom_hline(aes(yintercept = cp.best$slope, colour = "C2P"), show_guide = FALSE) + 
  scale_color_manual(name="Appraoch", breaks=c("C2P", "P2P", "CP2P"), values =  cols[c(1,3,2)]) + 
  scale_y_continuous(breaks = seq(0.4, 0.95, 0.05), "Test AUROC") +
  scale_x_continuous(breaks = seq(10, 150, by = 20), "# Number of Patient Samples in Training")

p4 <- p4 + theme(legend.direction = 'horizontal', 
      legend.position = 'top', 
      plot.margin = unit(c(5.1, 7, 4.5, 3.5)/2, "lines"), 
      text = element_text(size=15), axis.title.x=element_text(vjust=-1.5), axis.title.y=element_text(vjust=2))   
p4

如何仅在“ C2P”的图例中将符号更改为不带点的行，而又不影响“ P2P”和“ CP2P”的符号？