title: "ValueError: could not convert string to float" with Python ggplot library when making scatter plot
body: <p>Below is a pandas dataframe called 'grouped'. I want to make a scatter plot with Dayn on the X and ENTRIESn_hourly on the Y. I can make a bar chart no problem, but when I try to make a scatter plot I get this error: ValueError: could not convert string to float: Fri. </p>
  
  <pre><code>  Dayn  rain  ENTRIESn_hourly
  0   Fri     0      1336.012020
  1   Fri     1      1327.224794
  2   Mon     0      1195.493274
  etc. 
  </code></pre>
  
  <p><strong>Code</strong></p>
  
  <pre><code>plot = ggplot(grouped, aes('Dayn','ENTRIESn_hourly')) + \
         geom_point() + \
         ggtitle("Entries per day") + \
         xlab("Day of week") + \
         ylab("Average entries (millions)")
  return plot
  </code></pre>