title: How to remove an aesthetic from a ggplot2 legend
body: <p>I am looking for a way to hide one of the aestetic legends from the plot created with the code below.  To scale the point color by date, I had to convert the dates into numbers, and I'd rather not show the date legend on the plot.  On the other hand, the shape legend is important information to display.  I understand that <code>legend.position="none"</code> will completely remove the legend, but then that leaves me with the problem of how to communicate the meaning behind the shapes.</p>
  
  <pre><code>library(ggplot2)
  w&lt;-read.table("data.txt", header=TRUE)
  pt.data &lt;- w[w$dt==min(w$dt),]
  p &lt;- ggplot(data=w, aes(OAD,RtgValInt,color=dt,shape=Port)) +
       geom_jitter(size=3, alpha=0.75) +
       scale_colour_gradient(limits=c(min(w$dt), 
               max(w$dt)),
           low="#9999FF", high="#000066") +
       geom_point(data=pt.data, 
           color="red", size=3, aes(shape=Port))
  print(p)
  </code></pre>
  
  <p>The <code>data.txt</code> file includes the lines below.</p>
  
  <pre><code>Date          Port    OAD         RtgValInt   dt
  12/31/2010  Grp1    1.463771    1.833333    14974
  12/31/2010  Grp2    1.193307    2.071429    14974
  11/30/2010  Grp1    1.454115    1.833333    14943
  11/30/2010  Grp2    1.127755    2.071429    14943
  10/29/2010  Grp1    1.434965    2.000000    14911
  10/29/2010  Grp2    1.055758    2.071429    14911
  09/30/2010  Grp1    1.441773    2.000000    14882
  09/30/2010  Grp2    1.077799    2.071429    14882
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/ctTlR.jpg" alt="enter image description here"></p>