ggplot2 图例不起作用/添加手动图例

Question

I'm new to R. Legends and plotting seem to be more difficult than in Python. How can I change the graph to display each node as a different color in the legend? Now I have something like the picture. Thank you for your help.

library(ggplot2)
library(MASS)
library(car)
library(robustbase)

data("airquality")
# Select only Ozone and Temp variables
air = airquality[c("Ozone" , "Temp")]
# We need to remove NA from data set
air = na.omit(air)
air.center  = colMeans(air)
air.cov = cov(air)
rad  = sqrt(qchisq(p = 0.95 , df = ncol(air)))
ellipse <- ellipse(center = air.center , shape = air.cov , radius = rad ,
                        segments = 150 , draw = FALSE)

ellipse <- as.data.frame(ellipse)
colnames(ellipse) <- colnames(air)
# Finding distances
distances <- mahalanobis(x = air , center = air.center , cov = air.cov)
# Cutoff value for ditances from Chi-Sqaure Dist. 
# with p = 0.95 df = 2 which in ncol(air)
cutoff <- qchisq(p = 0.95 , df = ncol(air))
### Minimum Covariance Determinant (MCD)
Y_mcd <- covMcd(air)
# Robust estimate of location
Y_mcd$center
# Robust estimate of scatter
Y_mcd$cov
# Make elilipse 
ellipse_mcd <- data.frame(ellipse(center = Y_mcd$center,
                                  shape = Y_mcd$cov,
                                  radius= rad, 
                                  segments=100,draw=FALSE))
#the same names as in previous plot
colnames(ellipse_mcd) <- colnames(air)
plot_fig <- ggplot(air , aes(x = Ozone , y = Temp)) +
  geom_point(size = 2) +
  geom_polygon(data = ellipse , fill = "blue" , color = "blue" , alpha = 0.5,show.legend    =T)+
  geom_point(aes(air.center[1] , air.center[2],fill='Mahalanobis') , size = 5 , color = "blue") +
  geom_text(data=subset(air, distances > cutoff),
            aes(Ozone,Temp,label=row.names(air[distances > cutoff ,])),  hjust = 1 , vjust = -1.5 ,size = 3.5)+
  ylab("Temperature Values") + xlab("Ozone Values")+ggtitle("Mahalanobis distance")+ theme(
    legend.position = c(0.95, 0.15),
    legend.justification = c("right", "top")
  ) + geom_polygon(data=ellipse_mcd,aes(x = Ozone,y = Temp, colour='LINE2'), color="red", fill="red", 
                   alpha=0.3, inherit.aes = FALSE) +
  geom_point(aes(x = Y_mcd$center[1], y = Y_mcd$center[2],fill='MCD'),
             color = "red", size = 6)

plot_fig

Answer 1

The issue is that you mapped on the fill aesthetic and set the color of the points as arguments. Instead map the labels on the color aes and set the color values via scale_color_manual :

plot_fig <- ggplot(air , aes(x = Ozone , y = Temp)) +
  geom_point(size = 2) +
  geom_polygon(data = ellipse , fill = "blue" , color = "blue" , alpha = 0.5, show.legend =T)+
  geom_point(aes(air.center[1] , air.center[2], color = 'Mahalanobis'), size = 5) +
  geom_text(data=subset(air, distances > cutoff),
            aes(Ozone,Temp,label=row.names(air[distances > cutoff ,])),  hjust = 1 , vjust = -1.5 ,size = 3.5)+
  ylab("Temperature Values") + 
  xlab("Ozone Values")+
  ggtitle("Mahalanobis distance")+ 
  theme(
    legend.position = c(0.95, 0.15),
    legend.justification = c("right", "top")
  ) + 
  geom_polygon(data=ellipse_mcd,aes(x = Ozone,y = Temp, colour='LINE2'), color="red", fill="red", 
                   alpha=0.3, inherit.aes = FALSE) +
  geom_point(aes(x = Y_mcd$center[1], y = Y_mcd$center[2], color='MCD'), size = 6) +
  scale_color_manual(values = c(MCD = "red", Mahalanobis = "blue"))

plot_fig

Answer 2

If you map the Mahalanobis/MCD categorical variables to colour , then let fill be dependent on the mapped colour, the legend should sort itself out naturally and you can set the colours with scale_colour_manual() .

ggplot(air, aes(Ozone, Temp)) +
  geom_point(size = 2) +
  geom_polygon(
    data = ellipse, 
    aes(fill = after_scale(alpha(colour, 0.5)), colour = "Mahalanobis")
  ) +
  geom_polygon(data = ellipse_mcd,
               aes(fill = after_scale(alpha(colour, 0.3)), colour = "MCD")) +
  geom_point(aes(air.center[1], air.center[2], colour = "Mahalanobis"), 
             size = 5) +
  geom_point(aes(Y_mcd$center[1], Y_mcd$center[2], colour = "MCD"), size = 5) +
  geom_text(data = subset(air, distances > cutoff),
            aes(label = row.names(air[distances > cutoff, ])),
            hjust = 1, vjust = -1.5, size = 3.5) +
  scale_colour_manual(values = c("blue", "red")) +
  labs(x = "Ozone Values", y = "Temperature values", 
       title = "Mahalanobis distance") +
  theme(legend.position = c(0.95, 0.15),
        legend.justification = c("right", "top"))