title: R ggplot facet_wrap with different y-axis labels, one values, one percentages
body: <p>I'm plotting a time series value with its percentages using <code>facet_wrap</code> in ggplot:</p>
  
  <p>For the plot below, the upper plot is the value, and the lower plot is percentage change. And I would like the y-axis in the lower plot to be "%". Normally in ggplot I would do something like </p>
  
  <pre><code>+ scale_y_continuous(labels = scales::percent)
  </code></pre>
  
  <p>But since I'm using facet_wrap, how do I specify that I only want one of the 2 plots' y-axis label to be percentages?</p>
  
  <p><a href="https://i.stack.imgur.com/xSBXN.png" rel="noreferrer"><img src="https://i.stack.imgur.com/xSBXN.png" alt="enter image description here"></a></p>
  
  <p>P.S. Here is the code to generate this plot:</p>
  
  <pre><code>library(data.table)
  library(ggplot2)
  library(scales)
  library(dplyr)
  
  pct &lt;- function(x) {x/lag(x)-1}
  Dates = seq(from = as.Date("2000-01-01"), 
              to =as.Date("2018-10-01"), 
              by = "1 month")
  set.seed(1024)
  this_raw = data.frame(CM = Dates,
                        value = rnorm(n = length(Dates)),
                        variable = rep("FAKE",length(Dates)))
  this_diff = na.omit(as.data.table(this_raw %&gt;% 
                                      group_by(variable) %&gt;%
                                      mutate_each(funs(pct), c(value))))
  this_diff$type = "PerCng"
  this_raw$type = "RAW"
  plot_all = rbindlist(list(this_raw,this_diff))
  plot_all$type = factor(plot_all$type, levels = c("RAW", "PerCng"))
  
  out_gg = plot_all %&gt;%
    ggplot(aes(x=CM, y=value)) + 
    geom_line(color = "royalblue3") +
    theme(legend.position='bottom')+
    ggtitle("FAKE DATA") + 
    facet_wrap(~ type, scale = "free_y", nrow = 2,
               strip.position = "left", 
               labeller = as_labeller(c(RAW = "Original", PerCng = "% Change") ) )+
    scale_x_date(date_breaks = "12 month", date_labels = "%Y-%m", 
                 date_minor_breaks = "3 month")+
    ylab("")+
    theme(plot.title = element_text(hjust = 0.5,size = 12),  
          axis.text.x = element_text(size = 6,angle = 45, hjust = 1),
          axis.text.y = element_text(size = 6),  
          axis.title.y = element_text(size = 6)) + 
    theme(strip.background = element_blank(),
          strip.placement = "outside")+
    theme(legend.title=element_blank())
  print(out_gg)
  </code></pre>