title: Using `geom_line()` with X axis being factors
body: <p>Suppose I have a dataframe:</p>
  
  <pre><code>hist &lt;- data.frame(date=Sys.Date() + 0:13,
                     counts=1:14)
  </code></pre>
  
  <p>I want to plot the total count against weekday, using a <em>line</em> to connect the points.
  The following puts <em>points</em> on each value:</p>
  
  <pre><code>hist &lt;- transform(hist, weekday=factor(weekdays(date),
                                         levels=c('Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday', 'Sunday')))
  ggplot(hist, aes(x=weekday, y=counts)) + geom_point(stat='summary', fun.y=sum)
  </code></pre>
  
  <p>When I try to connect them with a line (<code>geom_line()</code>), ggplot complains about only having one data observation per group and hence is not able to draw a line between the points.</p>
  
  <p>I understand this - it's trying to draw one line for each weekday (factor level).</p>
  
  <p>How can I get ggplot to just pretend (for the purposes of the line only) that the weekdays are numeric? Perhaps I have to have another column <code>day_of_week</code> that is 0 for monday, 1 for tuesday, etc?</p>