title: ggplot XY scatter - how to change alpha transparency for select points?
body: <p>I have ~14,000 XY pairs to plot, and using <code>ggplot2</code> for this.</p>
  
  <p>Due to high numbers of points, I have had to use very low <code>alpha=0.025</code>.
  I want to highlight 7 XY points in a different color and more opaque, and have an accompanying text legend.</p>
  
  <p>Currently, my colors for the 7 special data points do not show up because they are also at <code>alpha=0.025</code>. How do I increase the opaqueness for just these points?</p>
  
  <p>Syntax I have so far is:</p>
  
  <pre><code>trial &lt;- ggplot(df, aes(x = df$SeqIdentityMean, 
                          y = df$SeqIdentityStdDev,
                          color = factor(df$PfamA_ID))) + 
              geom_point(alpha=0.025) + 
              labs(title="Mean Vs. standard deviation of PfamA seed lengths", 
                   x="Average Length (aa)",
                   y="Standard  Deviation of Length (aa)") + 
              theme(legend.title=element_blank(),
                    legend.key=element_rect(fill='NA'))
  </code></pre>