title: Hide missing dates on a plot
body: <p>I'm trying to plot data from 3 different years (2016-7-8) but I only have the summer months (basically no data from September to April).</p>
  
  <p>When I plot the data, the months with no data appear on the graph, leaving big ugly empty spaces</p>
  
  <p>When I pass the dates in factors, those spaces disappear. But then the dates do not appear on the x-axis anymore, I only get numbers ...</p>
  
  <p>Here is some of my data:</p>
  
  <pre><code>date    MD g/day    AM g/day
  26/04/2016  82  154
  27/04/2016  238 140
  28/04/2016  140 661
  29/04/2016  181 304
  30/04/2016  92  329
  07/07/2017  976 126
  08/07/2017  923 47
  09/07/2017  527 77
  10/07/2017  285 84
  11/07/2017  8704    155
  07/06/2018  3115    170
  08/06/2018  151 65
  09/06/2018  415 247
  10/06/2018  153 402
  11/06/2018  172 95
  12/06/2018  188 114
  
  </code></pre>
  
  <p>I transform my data to have MD and AM as factor levels</p>
  
  <pre><code>a &lt;- loads$MD.g.day
  b &lt;- loads$AM.g.day
  d &lt;- data.frame(date=loads$date, MD=a, AM=b)
  d$date &lt;- as.Date(loads$date, format='%d/%m/%Y')
  colnames(d) &lt;- c('date','MD','AM')
  e &lt;- rbind(data.frame(date=c(d$date), gday=c(d$MD), factor='MD'),
             data.frame(date=c(d$date), gday=c(d$AM), factor='AM'))
  </code></pre>
  
  <p>and then I plot using:</p>
  
  <pre><code>p &lt;- ggplot(data=e,aes(x=date))+ #select data
    geom_point(aes(y=gday*7/35, color=factor, shape=factor), data=e[e$factor=='MD', ], )+ #select and scale
    geom_line(aes(y=gday*7/35, color=factor), data=e[e$factor=='MD', ])+ #select and scale md
  
    geom_point(aes(y=gday, color=factor, shape=factor), data=e[e$factor=='AM', ])+ #select other compound
    geom_line(aes(y=gday, color=factor), data=e[e$factor=='AM', ])+ #select other compound
  
  
    scale_y_continuous(name = 'AM [g/day]\n',
                       sec.axis = sec_axis(~.*35/7, name = "MD [g/day]\n"), limits = c(0,7000))+  #add y-axis texts and secondary y-axis
    scale_x_date(date_labels = '%e %b %y', date_breaks='1 month')+  #arrange text for the x-axis
    scale_color_manual(values=c(MD='magenta', AM='light green'))+ #define colors
    scale_shape_manual(values=c(MD=21, AM=21))+ #define dot shapes
    scale_size_manual(values=c(MD=1.5, AM=2.5))+ #define dot sizes
    theme(axis.text.x = element_text(angle=90)), #turn text from the x-axis
  </code></pre>