title: Strings as tick values for rCharts NVD3 line chart
body: <p>I'm trying to use display categorical levels in the x-axis of my line chart. Say I have a data frame</p>
  
  <pre><code>mydata &lt;- data.frame(time = c("10am", "12pm", "3pm"), month = c("JAN", "JUL", "DEC"), value = 3:5)
  </code></pre>
  
  <p>and I want to plot <code>value ~ time</code> or <code>value ~ month</code>. I could first create a variable that puts time on a linear scale, e.g.</p>
  
  <pre><code>mydata$time_linear &lt;- 1:3
  </code></pre>
  
  <p>then try to plot</p>
  
  <pre><code>library(rCharts)
  p &lt;- nPlot(value ~ time_linear, data = mydata, type = "lineChart")
  p$chart(useInteractiveGuideline = TRUE)
  p
  </code></pre>
  
  <p>but I don't know how to force the labels.</p>
  
  <p>I have looked at <a href="https://stackoverflow.com/questions/18648565/how-to-set-a-nvd3-axis-to-use-strings-instead-of-numerical-values">How to set a nvd3 axis to use strings instead of numerical values ?</a> and <a href="https://stackoverflow.com/questions/23727627/nvd3-line-chart-with-string-values-on-x-axis">nvd3 line chart with string values on x-axis</a> but am not sure how it works in the context of <code>rCharts</code>. It seems like one way would be to define an array in javascript with the label names and index accordingly, and another would be to define a function with a bunch of <code>if/else</code> statements. The former method seems more elegant but I don't know how to conjure up an array in the right scope, and the latter would involve a nasty long anonymous function in <code>tickFormat</code></p>
  
  <p>What's the best way to go about this?</p>