title: Increasing minimum point size in ggplot geom_point
body: <p>I am using following data and code: </p>
  
  <pre><code>&gt; 
  &gt; dput(ddf)
  structure(list(xx = c("aa", "bb", "cc"), gp = c("one", "two", 
  "one"), yy = c(5L, 10L, 15L)), .Names = c("xx", "gp", "yy"), class = "data.frame", row.names = c(NA, 
  -3L))
  &gt; 
  &gt; 
  &gt; ddf
    xx  gp yy
  1 aa one  5
  2 bb two 10
  3 cc one 15
  &gt; 
  &gt; ggplot(ddf)+geom_point(aes(x=xx, y=yy, size=gp))
  &gt; 
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/96Ijc.png" alt="enter image description here"></p>
  
  <p>The smaller point size here is really very small and is hardly visible. It becomes even more obscure if it is colored. Can the smaller point size be increased so that it is clearly visible?</p>