title: R ggplot remove certain items from legend
body: <p>Is it possible to remove certain items from a legend created with ggplot? I have a plot that is faceted, and point sizes provide another dimension to the plot. Since the plot is faceted I do not need to have certain legend items since it is explained by the facet titles, but the legend is still relevant for the point size.</p>
  
  <p>In the plot below I would like to remove the "AREA" legend items since it is already explained by the faceting, but keep the "TOTAL_VOLUME" legend items that explain the point sizes.</p>
  
  <p><img src="https://i.stack.imgur.com/kie49.png" alt="a"></p>
  
  <p>Here is the code used to generate the plot:</p>
  
  <pre><code>library(data.table) # Import libraries
  library(ggplot2)
  library(scales)
  
  set.seed(1234) # Set Seed
  
  area.list &lt;- LETTERS[seq(1:7)] # 7 Possible areas
  date.list &lt;- seq(as.Date("2014/03/01"), by="month", length=13)
  
  # Build a random data set
  data &lt;- data.table(AREA = sample(area.list, 80, replace=TRUE),
                 DATE = sample(date.list, 80, replace=TRUE),
                 VOLUME = rnorm(n=80, mean=100000,sd=40000),
                 NON_CONFORMING_VOLUME = rnorm(n=80, mean=30000,sd=5000))
  # Summarise data by area and date
  data &lt;- data[, list(TOTAL_VOLUME=sum(VOLUME),
                  TOTAL_NC_VOLUME=sum(NON_CONFORMING_VOLUME)),
           by=list(AREA, DATE)]
  data$PERCENT_NC &lt;- data$TOTAL_NC_VOLUME / data$TOTAL_VOLUME * 100
  
  p &lt;- ggplot(data = data, aes(x = DATE, 
                            y = PERCENT_NC,
                            colour = AREA)) +
    geom_point(aes(size = TOTAL_VOLUME)) +
    geom_line() +
    facet_grid(. ~ AREA) +
    theme(legend.position="bottom", axis.text.x=element_text(angle=90,hjust=1)) +
    ggtitle("Percent Non-Conforming by Area by Month") +
    labs(x = "Month", y = "% Non-Conforming") +
    scale_size_continuous(labels = comma)
  
  plot(p)
  </code></pre>
  
  <p>I tried adding show_guide=FALSE to geom_point() but that removes both TOTAL_VOLUME and AREA.</p>
  
  <p>Thank you</p>