title: Scale the x-axes with quarterly date format
body: <p>I created a plot in R using the ggplot library:</p>
  
  <pre><code>library(ggplot2)
  ggplot(df, aes(x = yQ, y = value, group =1)) +
  geom_line(aes(color = variable), size = 1) +
     scale_color_manual(values = c("#00AFBB", "#E7B800"))
  </code></pre>
  
  <p>I got the plot that I want but the only problem is that variable, <code>yQ</code> values have the format:</p>
  
  <pre><code> 1990Q1
   1900Q2
   1990Q3
   1990Q4
   ......
   ......
   2017Q1
   2017Q2
   2017Q3
   2017Q4
  </code></pre>
  
  <p>and because there are many years, the x-axis label cannot show all the dates clearly (they overlapped).
  Therefore, I want the x-axis label to show only <code>Q1</code> and <code>Q3</code> for every 5 years.
  So I want the x-axis to be something like this:</p>
  
  <pre><code>    1990Q1   1990Q3   1995Q1   1995Q3 ......  2015Q1 2015Q3
  </code></pre>
  
  <p>I tried to use <code>scale_x_date</code> but my dates are not in date format (e.g. 1990Q1) and therefore this does not work. How can I fix it?</p>