R ggplot: How can I create conditional labeling for a continuous axis ticks

问题

I would like to conditionally alter the color/face/etc of a continuous tick mark label using logic instead of hard coding. For example:

library(tidyverse)
library(viridis)
xx=rpois(1000,lambda = 40)
y=density(xx,n=3600,from=0)

ggplot(data.frame(x = y$x, y = y$y), aes(x, y)) + 
  geom_line() + 
  geom_segment(aes(xend = x, yend = 0, colour = y)) + 
  scale_color_viridis() +
  labs(y='Density',x='Count',colour='Density')+
  geom_vline(xintercept=40,color='red') +
  scale_x_continuous(breaks=c(0,40,seq(25,100,25)),limits=c(0,100))+
  theme(axis.text.x = element_text(face=c('plain','bold',rep('plain',4)),
                                   color=ifelse(y$x==60,'red','black')))

So in my example above, hard coding is seen in the face function and that works (I can do the same thing for color). It's not a big deal here since I only have a few breaks. In future scenarios, though, I may have significantly more breaks or a less simple coloring scheme needed for my axes. Is there a more efficient solution to create this labeling based on conditional logic? My first attempt is seen in color function but that does not work . The issue is identifying the object to use in the logical statement. Maybe there is a behind the scenes variable I can call for the breaks (similar to using ..level.. for a density plot). If that's the case, bonus points if you can teach me how to find it/figure that out on my own

回答1:

Quick thanks to Djork for teaching me some QnA etiquette...

I was able to solve this problem by defining my break points outside of ggplot and then using ifelse logic within the theme function to create my desired outcome. The updated (and working) example of my original code is below:

library(tidyverse)
library(viridis)
xx=rpois(1000,lambda = 40)
y=density(xx,n=3600,from=0)

med_x=median(xx)
breakers = c(seq(0,100,25),med_x)

ggplot(data.frame(x = y$x, y = y$y), aes(x, y)) + 
  geom_line() + 
  geom_segment(aes(xend = x, yend = 0, colour = y)) + 
  scale_color_viridis() +
  labs(y='Density',x='Count',colour='Density')+
  geom_vline(xintercept=40,color='red') +
  scale_x_continuous(breaks=breakers,limits=c(0,100))+
  theme(axis.text.x = element_text(face=ifelse(breakers==med_x,'bold','plain'),
                                   color=ifelse(breakers==med_x,'red','black')))

I haven't tried this on more complicated logic yet but I assume that this approach will work across all logical formatting.

来源：https://stackoverflow.com/questions/49076565/r-ggplot-how-can-i-create-conditional-labeling-for-a-continuous-axis-ticks