title: Error in plot, formula missing when using svm
body: <p>I am trying to plot my svm model.</p>
  
  <pre><code>library(foreign)
  library(e1071)
  
  x &lt;- read.arff("contact-lenses.arff")
  #alt: x &lt;- read.arff("http://storm.cis.fordham.edu/~gweiss/data-mining/weka-data/contact-lenses.arff")
  model &lt;- svm(`contact-lenses` ~ . , data = x, type = "C-classification", kernel = "linear")
  </code></pre>
  
  <p>The contact lens arff is the inbuilt data file in weka. </p>
  
  <p>However, now i run into an error trying to plot the model. </p>
  
  <pre><code> plot(model, x)
  Error in plot.svm(model, x) : missing formula.
  </code></pre>