title: How to add a legend for the secondary axis ggplot
body: <p>I am trying to add a legend for the secondary axis in the following code</p>
  
  <pre><code>library(ggplot2)
  library(dplyr)
  library(reshape2)
  
  df = data.frame(period = seq(as.POSIXct("2017-01-01"),as.POSIXct("2017-12-01"), by = "month"), 
                  b = c(100, 110, 105, 200, 210, 190, 180, 170, 165, 175, 140, 145),
                  c = c(120, 130, 150, 170, 250, 160, 130, 120, 110, 130, 120, 170),
                  d = c(1060, 1180, 1050, 2070, 2150, 1900, 1850, 1070, 1605, 1750, 1460, 1250)) %&gt;% 
               mutate(period = factor(period))
  
  df_bar = melt(df, id.vars = "period", measure.vars = c("b", "c", "d")) %&gt;% filter(variable != "d")
  
  df_line = df %&gt;% select(period, d)
  
  ggplot(data = df_bar, aes(x = period, y = value, fill = variable)) + 
   geom_bar(stat = "identity", position = "dodge") +
   theme(axis.text.x = element_text(angle = 90, hjust = 1)) +
   theme(axis.text=element_text(size=9), 
   axis.title=element_text(size=14,face="bold")) +
   ylab("primary axis") +
   geom_line(data = df_line, aes(x = period, y = (d)/10, group = 1), inherit.aes = FALSE) + 
   geom_point(data = df_line, aes(x = period, y = (d)/10, group = 1), inherit.aes = FALSE) +
   scale_y_continuous(sec.axis = sec_axis(~.*10, name = "secondary axis"))
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/ER080.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ER080.png" alt="plot"></a></p>
  
  <p>I would like to have a legend for the line graph as well.</p>