title: Conditional Formatting ggplot label ticks without explicitly repeat all tick breaks vector
body: <p>I want to create a nice plot to show the number of clusters I choose, which is 11.</p>
  
  <pre><code>RS &lt;- structure(list(k = 2:24, value = c(0.144146119842721, 0.222206168029977, 
  0.291678571330934, 0.358919047522653, 0.426695606507329, 0.483301229692586, 
  0.533728497594114, 0.580550588772305, 0.588601124711909, 0.635271587964058, 
  0.650036125839732, 0.662220837971202, 0.675275766505185, 0.685226878333768, 
  0.696300002606587, 0.705498413223437, 0.709250552710995, 0.716976400569355, 
  0.72489699345261, 0.735887738035583, 0.741940077625176, 0.742248532545676, 
  0.75185453741879)), .Names = c("k", "value"), row.names = c(NA, 
  -23L), class = "data.frame")
  
  
      ggplot(RS)  + 
  ggtitle("Root-mean square standard deviation error") + 
  geom_line(aes(x = k, y = value), size = 1) + 
  geom_point(aes(x = k, y = value), size = 2) + 
  geom_segment(aes(x = RS[RS$k == 11, "k"], xend =  RS[RS$k == 11, "k"], y = min(RS[ , "value"]), yend = RS[RS$k == 11, "value"]), linetype = 2)  + 
  scale_x_continuous(breaks = c(2, 5, 10, 11, 15, 20)) + theme_classic() +         
  theme(axis.text.y = element_text(size=14), axis.text.x = element_text(size = 14), axis.title.x = element_text(size = 14), axis.title.y = element_text(size = 14))
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/723S5.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/723S5.png" alt="enter image description here"></a></p>
  
  <p>How can I format the xtick value <code>11</code> <strong>without explicitly writing the entire vector like in this solution</strong>?</p>
  
  <pre><code>ggplot(RS)  + 
          ggtitle("Root-mean square standard deviation error") +
          geom_line(aes(x = k, y = value), size = 1) +
          geom_point(aes(x = k, y = value), size = 2) +
          geom_segment(aes(x = RS[RS$k == 11, "k"],
                           xend =  RS[RS$k == 11, "k"],
                           y = min(RS[ , "value"]),
                           yend = RS[RS$k == 11, "value"]),
                       linetype = 2)  +
          scale_x_continuous(breaks = c(2, 5, 10, 11, 15, 20, 24)) +
          theme_classic() +
          theme(axis.text.y = element_text(size=14),
                axis.text.x = element_text(size = 14,
                                           face=ifelse(c(2,5,10,11,15,20, 24) == 11,'bold','plain'),
                                           color =ifelse(c(2,5,10,11,15,20, 24) == 11,'red','black')),
                axis.title.x = element_text(size = 14),
                axis.title.y = element_text(size = 14))  
  </code></pre>