title: R ggplot custom grid
body: <p>I want to plot values versus dates and adjust the grids of a graph so that on the x axis the main grid falls on every Sunday, and keep the minor grid for everyday. On the y axis i would like to have the major grid at every unit, and no minor grid.</p>
  
  <p>The following example, with default grid, places the major grid every 2 units on each axis, and the minor grid every unit (which is actually OK if we can change only the major grid). How can I solve this ?</p>
  
  <pre><code>require(ggplot2)
  data &lt;- data.frame(date = seq(as.Date("2016-05-02"),as.Date("2016-05-16"),2),
                age_in_days = seq(1,15,2)))
  
  ggplot(data=data,aes(x=date,y=age_in_days)) + geom_line()
  </code></pre>