title: Bar Plot x-axis order in R ggvis
body: <p>I am experimenting to use ggvis for my daily report. One issue I came across is that for a dataset that I intended to plot x-axis as in the data order. However, the actual plot shows up in the alphabet order. </p>
  
  <p>Is there any hidden parameters to change x-axis order for Bar chart in R?</p>
  
  <pre><code>Step    LER
  Pre 3.2
  DS  2.8
  SiARC   2.2
  OPL 1.9
  ILD 1.6
  Oxide   1.5
  
  library(ggvis)
  ler &lt;- read.csv("shinyapps/EUV/data/by_step_LER.csv")
  ler %&gt;% ggvis(x = ~Step, y = ~LER) %&gt;% layer_bars()
  </code></pre>