title: Getting ggvis::export_png() working
body: <h1>Goal</h1>
  
  <p>Export a <code>ggvis</code> figure as a PNG file (for inclusion in an .Rmd document).</p>
  
  <h1>Problem</h1>
  
  <p>I know essentially nothing about Node.js, other than that it is great and I should know more.</p>
  
  <p>I get as far as:</p>
  
  <pre><code>library(ggvis)
  mtcars %&gt;% ggvis(~mpg, ~wt) %&gt;% export_png()
  Writing to file plot.png
  Guessing layer_points()
  
  module.js:340
      throw err;
            ^
  Error: Cannot find module 'd3'
      at Function.Module._resolveFilename (module.js:338:15)
      at Function.Module._load (module.js:280:25)
      at Module.require (module.js:364:17)
      at require (module.js:380:17)
      at Object.&lt;anonymous&gt; (/usr/local/src/vega/index.js:11:6)
      at Module._compile (module.js:456:26)
      at Object.Module._extensions..js (module.js:474:10)
      at Module.load (module.js:356:32)
      at Function.Module._load (module.js:312:12)
      at Module.require (module.js:364:17)
  </code></pre>
  
  <hr>
  
  <h1>Setup</h1>
  
  <h2>Platform</h2>
  
  <ul>
  <li>OS X Mavericks (10.9.5)</li>
  <li>RStudio (0.98.945)</li>
  <li>Used <code>devtools::install_github("hadley/ggvis")</code> to install <code>ggvis</code> (0.3.0.9001) and dependencies</li>
  <li>Cloned <code>https://github.com/trifacta/vega</code> to <code>/usr/local/src/vega</code></li>
  <li>Symlinked <code>./bin/vg2png -&gt; /usr/local/src/vega/bin/vg2png</code></li>
  </ul>
  
  <h2>sessionInfo()</h2>
  
  <pre><code>sessionInfo()
  R version 3.1.0 (2014-04-10)
  Platform: x86_64-apple-darwin13.1.0 (64-bit)
  
  locale:
  [1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
  
  attached base packages:
  [1] graphics  grDevices utils     datasets  stats     methods   base     
  
  other attached packages:
  [1] knitr_1.6        pander_0.3.8     ggvis_0.3.0.9001 lubridate_1.3.3  dplyr_0.2.0.9001 plyr_1.8.1       stringr_0.6.2    ggplot2_1.0.0    devtools_1.5    
  
  loaded via a namespace (and not attached):
   [1] assertthat_0.1   bitops_1.0-6     caTools_1.17     colorspace_1.2-4 DBI_0.3.0        digest_0.6.4     evaluate_0.5.5   formatR_0.10     grid_3.1.0      
  [10] gtable_0.1.2     htmltools_0.2.4  httpuv_1.3.0     httr_0.5.0.9000  jsonlite_0.9.11  lazyeval_0.1.1   magrittr_1.0.1   MASS_7.3-33      memoise_0.2.1   
  [19] munsell_0.4.2    parallel_3.1.0   proto_0.3-10     Rcpp_0.11.2      RCurl_1.95-4.3   reshape2_1.4     RJSONIO_1.3-0    scales_0.2.4     shiny_0.10.1    
  [28] tools_3.1.0      whisker_0.3-2    xtable_1.7-3    
  </code></pre>