title: Circlize chord diagram with multiple levels of data
body: <p>I am finding myself a bit stuck,
  I want to show flows between regions for trafficked species via a chord diagram on circlize, but am unable to work out how to plot when column 1 and 2 represent the "connection", column 3 is the "factor" of interest and column 4 are the values.
  I have included a sample of data below (yes I am aware indonesia is a region), as you can see each species is not unique to a particular region. I would like to produce a plot similar to the one included below but replace the "countries" with "species" for each region. Is this possible to do?</p>
  
  <pre><code>import_region    export_region  species                flow
  North America    Europe         Acanthosaura armata     0.0104
  Southeast Asia   Europe         Acanthosaura armata     0.0022
  Indonesia        Europe         Acanthosaura armata     0.1971
  Indonesia        Europe         Acrochordus granulatus  0.7846
  Southeast Asia   Europe         Acrochordus granulatus  0.1101
  Indonesia        Europe         Acrochordus javanicus   2.00E-04
  Southeast Asia   Europe         Acrochordus javanicus   0.0015
  Indonesia        North America  Acrochordus javanicus   0.0024
  East Asia        Europe         Acrochordus javanicus   0.0028
  Indonesia        Europe         Ahaetulla prasina       4.00E-04
  Southeast Asia   Europe         Ahaetulla prasina       4.00E-04
  Southeast Asia   East Asia      Amyda cartilaginea      0.0027
  Indonesia        East Asia      Amyda cartilaginea      5.00E-04
  Indonesia        Europe         Amyda cartilaginea      0.004
  Indonesia        Southeast Asia Amyda cartilaginea      0.0334
  Europe           North America  Amyda cartilaginea      4.00E-04
  Indonesia        North America  Amyda cartilaginea      0.1291
  Southeast Asia   Southeast Asia Amyda cartilaginea      0.0283
  Indonesia        West Asia      Amyda cartilaginea      0.7614
  South Asia       Europe         Amyda cartilaginea      2.8484
  Australasia      Europe         Apodora papuana         0.0368
  Indonesia        North America  Apodora papuana         0.324
  Indonesia        Europe         Apodora papuana         0.0691
  Europe           Europe         Apodora papuana         0.0106
  Indonesia        East Asia      Apodora papuana         0.0129
  Europe           North America  Apodora papuana         0.0034
  East Asia        East Asia      Apodora papuana         2.00E-04
  Indonesia        Southeast Asia Apodora papuana         0.0045
  East Asia        North America  Apodora papuans         0.0042
  </code></pre>
  
  <p>example of diagram similar to what I would like, please click link below:
  <a href="http://i.stack.imgur.com/MXYKU.png" rel="nofollow">chord diagram</a></p>