title: ddply error "unique() applies only to vectors" with data frame
body: <p>I'm trying to use <code>ddply</code> in a data frame (created using <code>melt</code>) as the following:</p>
  
  <pre><code>head(MoP_top50_ftrt_htmp_melt)
                                                                       Taxon variable     value
  1                                  K03406; methyl-accepting chemotaxis protein     Coal 0.2748281
  2                          K02014; iron complex outermembrane recepter protein     Coal 0.2676633
  3 K03296; hydrophobic/amphiphilic exporter-1 (mainly G- bacteria), HAE1 family     Coal 0.2516787
  4                         K06147; ATP-binding cassette, subfamily B, bacterial     Coal 0.2592270
  5                                                         K00936; [EC:2.7.3.-]     Coal 0.2278227
  6                                        K08300; ribonuclease E [EC:3.1.26.12]     Coal 0.2528503
  </code></pre>
  
  <p>And I'm trying to get it ready to <code>ggplot2</code> with proportions included.</p>
  
  <p>For that I'm using:</p>
  
  <pre><code>require(plyr)
  MoP_top50_ftrt_htmp_melt &lt;- ddply(MoP_top50_ftrt_htmp_melt, .(sample), transform, value=(value/sum(value))*100)
  </code></pre>
  
  <p>But getting this error:</p>
  
  <blockquote>
    <p>Error in unique.default(x) : unique() applies only to vectors</p>
  </blockquote>
  
  <p>Does this have to do with the semicolons and spaces in the Taxon column?</p>