title: geom_point smallest point size is not in proportion
body: <p>I am visualizing a variable using point size.
  My problem is that the first (smallest) point size (in this case "70") is significantly (not proportionally) smaller than the rest. (See the legend for CP)</p>
  
  <p><a href="https://i.stack.imgur.com/MEHMV.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/MEHMV.png" alt="Notice that the point size for &quot;70&quot; is very small"></a></p>
  
  <p>This is the code I'm using:</p>
  
  <pre><code>ggplot(data=testdata, aes(x=variable, y=value, group=ID, size=CP)) + geom_point()
  </code></pre>
  
  <p>On this data:</p>
  
  <pre><code>   ID  CP variable value
  1  C1  70        A    76
  2  C2  95        A    20
  3  C3 100        A    30
  4  C4  NA        A    17
  5  C1  70        B    36
  6  C2  95        B    10
  7  C3 100        B    51
  8  C4  NA        B    41
  9  C1  70        C    89
  10 C2  95        C     1
  11 C3 100        C    94
  12 C4  NA        C     6
  13 C1  70        D   100
  14 C2  95        D    98
  15 C3 100        D    16
  16 C4  NA        D    33
  </code></pre>
  
  <p>Now, when I'm trying to adjust point sizes R doesn't like it:</p>
  
  <pre><code>&gt; testplot = ggplot(data=testdata, aes(x=variable, y=value, group=ID, size=CP)) + geom_point()
  &gt; testplot = testplot + scale_size_discrete(range=c(3,5))
  Warning message:
  Using size for a discrete variable is not advised. 
  &gt; testplot
  Error: Continuous value supplied to discrete scale
  </code></pre>
  
  <p>And no plot is produced. Same for this:</p>
  
  <pre><code>&gt; testplot = ggplot(data=testdata, aes(x=variable, y=value, group=ID, size=CP)) + geom_point()
  &gt; testplot = testplot + scale_size_continuous(to=c(3,5))
  Error in scale_size_continuous(to = c(3, 5)) : 
    unused argument (to = c(3, 5))
  &gt; testplot
  Error: Continuous value supplied to discrete scale
  </code></pre>