title: How can I force ggplot to show more levels on the legend?
body: <p>I'm trying to create a complex ggplot plot but some things don't work as expected.</p>
  
  <p>I have extracted the problematic part, the creation of points and its associated legend.</p>
  
  <pre><code>library(data.table)
  library(ggplot2)
  lev &lt;- c("A", "B", "C", "D") # define levels.
  bb &lt;- c(40, 30,20,10,5)/100 # define breaks.
  ll &lt;- c("40%","30%","20%","10%","5%") # labels.
  # Create data
  nodos &lt;- data.table(event = c("A", "B", "D", "C", "D"), ord = c(1, 2, 3, 3, 4),
  NP = c(0.375, 0.25, 0.125, 0.125, 0.125))
  
  
  ggplot() + geom_point(data=nodos,aes(x=ord, 
   y=event, size=NP), color="black", shape=16) +
   ylim(lev)  + scale_size_continuous(name="Prop.",
   breaks=bb, labels=ll, range=c(0,6))+ 
   scale_x_continuous(limits=c(0.5, 4.5),
   breaks=seq(1,4,1))
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/kVIgo.jpg" rel="noreferrer"><img src="https://i.stack.imgur.com/kVIgo.jpg" alt="enter image description here"></a></p>
  
  <p>As you can see, no matter what breaks and labels I use I'm not able to force ggplot to paint a legend containing 0% or 10%. 
  scale_size_continuous keeps creating just two elements.<br>
  And the smaller points are very badly scaled.</p>
  
  <p>I have also tried with scale_scale_area, but it doesn't work either.</p>
  
  <p>I'm using R 3.4.2 and ggplot2 2.2.1 (also tried the latest github version).</p>
  
  <p>How can I get it?</p>