title: scale_x_discrete not displayed on the plot
body: <p>I have a data set that looks like this:</p>
  
  <pre><code>df &lt;- data.frame(
    EventMonth = c('2015-01', '2015-02', '2015-03', '2015-04', '2015-05', '2015-06', '2015-07', '2015-08', '2015-09', '2015-10', '2015-11', '2015-12',
                   '2016-01', '2016-02', '2016-03', '2016-04', '2016-05', '2016-06', '2016-07', '2016-08', '2016-09', '2016-10', '2016-11', '2016-12'),
    EventQuarter = c('2015-Q1', '2015-Q1', '2015-Q1', '2015-Q2', '2015-Q2', '2015-Q2', '2015-Q3', '2015-Q3', '2015-Q3', '2015-Q4', '2015-Q4', '2015-Q4',
                     '2016-Q1', '2016-Q1', '2016-Q1', '2016-Q2', '2016-Q2', '2016-Q2', '2016-Q3', '2016-Q3', '2016-Q3', '2016-Q4', '2016-Q4', '2016-Q4'),
    score = c(2.59, 2.58, 2.82, 2.60, 2.69, 2.76, 2.68, 2.65, 2.58, 2.51, 2.90, 2.50, 2.65, 2.87, 2.48, 2.59, 2.83, 2.63, 2.73, 2.41, 2.63, 2.60, 2.64, 2.51),
    stringsAsFactors = F
  )
  </code></pre>
  
  <p>I am using a command like below to show line graph:</p>
  
  <pre><code>ggplot(df, aes(EventMonth, score)) +
    geom_line(stat='identity', group = 1) +
    theme_few() + ylim(0, survey[item == col, max.val])  +
    ylab(survey[item == col, title]) +
    theme(axis.text.x = element_text(angle=90)) +
    stat_smooth(aes(as.integer(as.factor(EventMonth)), score), method='loess')
  </code></pre>
  
  <p>This works beautifully, but there are too many tick marks on the X axis. I would like to instead show only one tick mark per quarter.</p>
  
  <p>As a starting point, I tried adding this to the command, to just show some arbitrary values for each quarter:</p>
  
  <p><code>+ scale_x_discrete('Event Quarter', c(2, 5, 8, 11), c('a', 'b', 'c', 'd')</code></p>
  
  <p>But this just makes the entire x-axis ticks and labels disappear, without showing a, b, c, d in places that I expect to see them. What am I doing wrong?</p>
  
  <p>Also, my x axis is an integer axis, so I tried using scale_x_continuous too but couldn't get that to work either. It complained about me supplying discrete values for a continuous scale: <code>scale_x_continuous('Event Quarter', c(2, 5, 8, 11), c('a', 'b', 'c', 'd'))</code></p>