title: Scatter plot of variables in a column of a tibble using dplyr & ggplot2
body: <p>I have the following tibble that would like to use to make a scatter plot (using ggplot2) of logcpm values of AA_Colon vs. BA_Colon matched by gene.</p>
  
  <pre><code>             gene   sample     logcpm
               &lt;chr&gt;    &lt;chr&gt;      &lt;dbl&gt;
   1 ENSG00000169903 AA_Colon 0.31536340
   2 ENSG00000145321 AA_Colon 0.19735593
   3 ENSG00000171560 AA_Colon 0.00000000
   4 ENSG00000171557 AA_Colon 0.19735593
   5 ENSG00000106327 AA_Colon 0.06882901
   6 ENSG00000228278 AA_Colon 0.13452328
   7 ENSG00000138115 AA_Colon 0.31536340
   8 ENSG00000148702 AA_Colon 0.00000000
   9 ENSG00000140107 AA_Colon 0.00000000
  10 ENSG00000197723 AA_Colon 0.00000000
  11 ENSG00000169903 BA_Colon 1.14724849
  12 ENSG00000145321 BA_Colon 0.08113901
  13 ENSG00000171560 BA_Colon 0.36654820
  14 ENSG00000171557 BA_Colon 0.23088996
  15 ENSG00000106327 BA_Colon 0.08113901
  16 ENSG00000228278 BA_Colon 0.08113901
  17 ENSG00000138115 BA_Colon 0.42987550
  18 ENSG00000148702 BA_Colon 0.00000000
  19 ENSG00000140107 BA_Colon 0.00000000
  20 ENSG00000197723 BA_Colon 0.08113901
  </code></pre>
  
  <p>Currently, I'm doing (which works):</p>
  
  <pre><code>tibble %&gt;%
      spread(key = sample, value = logcpm) %&gt;%
      ggplot(aes(x = AA_Colon, y = BA_Colon)) +
      geom_point()
  </code></pre>
  
  <p>But, I'm wondering if there's a more elegant way to directly work with the tidy format and extract the two vectors to plot instead of spreading the data into two columns.</p>