title: ggplot r error Discrete value supplied to continuous scale
body: <p>I have a list as attached. </p>
  
  <pre><code>    Site Name  5833             count  ratio
  1   Boston     0 days per week  1      0.01428571
  2   Boston     1 day per week   6      0.08571429
  3   Boston     2 days per week  11     0.15714286
  4   Boston     3 days per week  5      0.07142857
  5   Seattle    0 days per week  2      0.02857143
  6  Seattle     1 day per week   6      0.08571429
  7  Seattle     2 days per week  10     0.14285714
  8  Seattle     3 days per week  8      0.11428571
  </code></pre>
  
  <p>when I use ggplot - I have to use name &lt;- "5833" since this will be put into for loop later.</p>
  
  <pre><code>name &lt;- "5833"
  ggplot(com_agg,aes(x=com_agg[,c(name)], y=ratio,fill=`Site Name`))+
  geom_bar(stat = "identity", position = "dodge")
  </code></pre>
  
  <p>I get error:</p>
  
  <pre><code>"Don't know how to automatically pick scale for object of type tbl_df/tbl/data.frame. 
  Defaulting to continuous. Error: Discrete value supplied to continuous scale"
  </code></pre>
  
  <p>I noticed the selection of column name for this dataframe have to be included in a pair of backtick automatically in R. it may be due to the space or special name in the columns. so I can't simply use name of column in ggplot.</p>