I have prepared sample data. I want to add a legend for this plot based on the error bar color. I tried to add legend manually, but it is not working. Thank you for the hlep.

data = data.frame(x = c(5,  10, 15, 20, 25, 30, 35, 40, 50),
                  Y1 = c(179,   212,    201,    204,    220,    181,    176,    219,    182),
                  SD1 = c(58,   93, 74, 55, 59  ,56,    53, 62, 62),
                  Y2 = c(273,   267,    329,    329,    386,    401,    399,    350,    274),
                  SD2 = c(107,  85, 141,    126,    94, 101,    65, 65, 58)) 
Y21 = data$Y2/5
SD21 = data$SD2/5
data = cbind(data, Y21, SD21)

ggplot(data=data,aes(x = x ,y=Y1)) 
  geom_errorbar(data=data,aes(ymin=Y1-SD1,ymax=Y1 SD1), colour="orange", width = 0.9, size = 1.5, linetype = "solid") 
  geom_point(aes(y=Y1), color = "black") 
  geom_errorbar(data=data,aes(ymin=Y2-SD2,ymax=Y2 SD2),color="blue", width = 0.9, size = 1.5, linetype = "solid") 
  scale_y_continuous("first y axis", sec.axis = sec_axis(Y2~ .*(5)  , name = "second y axis" )) 
  geom_point(aes(y=Y2), color = "black") 
  expand_limits(x = 0, y = 0)

CodePudding user response：

You can add a legend across multiple geoms by

1. setting color = "label" within aes() for each geom, where "label" is a unique label for each geom; then
2. add scale_color_manual() (or scale_linetype_manual(), etc, depending on the aesthetic), and set the values arg to a named vector whose names correspond to the geom labels, and values correspond to the colors (or linetypes, or whatever) wanted.

ggplot(data = data,aes(x = x, y = Y1))  
  geom_errorbar(
    aes(ymin = Y1 - SD1, ymax = Y1   SD1, color = "Y1"), 
    width = 0.9, 
    size = 1.5
  ) 
  geom_point(aes(y = Y1), color = "black")  
  geom_errorbar(
    aes(ymin = Y2 - SD2, ymax = Y2   SD2, color = "Y2"), 
    width = 0.9, 
    size = 1.5
  )  
  geom_point(aes(y = Y2), color = "black")  
  scale_y_continuous(
    "first y axis", 
    sec.axis = sec_axis(Y2 ~ .*(5), name = "second y axis")
  )  
  scale_color_manual("legend title", values = c(Y2 = "blue", Y1 = "orange"))  
  expand_limits(x = 0, y = 0)

CodePudding user response：

Often, if you find yourself making multiples of the same geom with manual aesthetics, it's a sign you should

If you really do need to or prefer to use separate geoms, though, I've described how to generate a legend across multiple geoms in a separate answer.