title: Plot gvisMotionChart with timevar as month and year
body: <p>I have a data frame <code>df</code> with a column </p>
  
  <pre><code>df$year_month = "2011-1" "2011-02" "2011-03" ...
  </code></pre>
  
  <p>I wanted to use this as the <code>timevar</code> argument of the <code>gvisMotionChart</code> function. I tried:</p>
  
  <pre><code>library(zoo)
  df$year_month = as.yearmon(df$year_month)
  </code></pre>
  
  <p>However, when I tried plotting the graph, I received the following error:</p>
  
  <blockquote>
    <p>The timevar has to be of numeric or Date format. Currently is it
    yearmon</p>
  </blockquote>
  
  <p>When plotting <code>gvisMotionChart</code> is it possible to have the time variable displaying the values:</p>
  
  <pre><code>"Jan 2011" "Feb 2011" ...
  </code></pre>
  
  <p>As Roland suggests, I could convert to numeric e.g. <code>201101, 201102, ...</code> but then presumably the time variable would display the values:</p>
  
  <pre><code>"201101" "201102" ...
  </code></pre>