Overlaying two geom_contours with specified breaks shows incorrect line types-CodePudding

Reproducible R code

library(ggplot2)
library(metR)
v <- reshape2::melt(volcano)
g <- ggplot(v, aes(Var1, Var2))  
  geom_contour(aes(z = value), color = "white", size = 1.25,
               breaks = c(100, 110, 120, 130, 140, 150, 160, 170, 180, 190))  
  geom_contour(aes(z = value, linetype = ifelse(value <= 130, "solid", "dash")),
               color = "black", size = 0.45,
               breaks = c(100, 110, 120, 130, 140, 150, 160, 170, 180, 190),
               show.legend = F)  
  geom_text_contour(aes(z = value))
g

When I overlay a geom_contour with a white background and another geom_contour with an ifelse function for different line types depending on value, the plotting procedure shows broken contours, close to the specified ifelse value of 130, as shown in the plot below.

Can someone recommend a solution? I suspect it has to do with the interpolation method.

Reproducible code without the metR library (used to plot contour labels on above plot)

library(ggplot2)
v <- reshape2::melt(volcano)
g <- ggplot(v, aes(Var1, Var2))  
  geom_contour(aes(z = value), color = "white", size = 1.25,
               breaks = c(100, 110, 120, 130, 140, 150, 160, 170, 180, 190))  
  geom_contour(aes(z = value, linetype = ifelse(value <= 130, "solid", "dash")),
               color = "black", size = 0.45,
               breaks = c(100, 110, 120, 130, 140, 150, 160, 170, 180, 190),
               show.legend = F)
g

CodePudding user response：

In short, there is no way to use ifelse successfully with the geom.contour function. The correct usage is by plotting both contours individually. Please note that the error is purely caused by your ifelse function, not by overlaying two plots, as you assumed in your title.

library(reshape2)
library(ggplot2)
v <- reshape2::melt(volcano)
g <- ggplot(v, aes(Var1, Var2))  
  geom_contour(aes(z = value, linetype = "solid"),
               color = "black", size = 0.45,
               breaks = c(140, 150, 160, 170, 180, 190),
               show.legend = F)  
  geom_contour(aes(z = value, linetype = "dashed"),
               color = "black", size = 0.45,
               breaks = c(100, 110, 120, 130),
               show.legend = F)
g

But, I still wonder if there is a way to do this successfully with the ifelse function?