title: Changing tick intervals when x axis values are dates
body: <p>Apologies if this is easy to solve, but I can't get my head around it.</p>
  
  <p>I have this data frame:</p>
  
  <pre><code>&gt; aaci
           Date    Plate.1    Plate.2   Plate.3   Plate.4    Plate.5     Plate.6   Plate.7    Plate.8   Average         SE Species
  1  2014-06-19  0.0000000  0.0000000  0.000000  0.000000  0.0000000   0.0000000  0.000000  0.0000000  0.000000  0.0000000      aa
  7  2014-08-04  7.0057778 11.0000000 24.269333 10.439111 28.5724444  92.4604444  5.584000 55.5448889 29.359500 10.7354126      aa
  13 2014-09-17 84.4075556 59.2493333 62.664444 38.147556 73.8417778  93.5208889 72.782667 94.5164444 72.391333  6.7116450      aa
  19 2014-10-16 56.9840000 64.9733333 45.124444 38.817333 56.2031111  76.4613333        NA 85.4017778 60.566476  6.2339579      aa
  25 2014-11-14 75.7146667 72.3604444 62.126222 20.095111 73.9520000  83.8688889        NA 61.5466667 64.237714  7.9248240      aa
  31 2014-12-12 61.6942222 65.4386667 46.337778 20.824444         NA  92.4413333        NA 76.3115556 60.508000 10.1172930      aa
  37 2015-02-12 63.0524444 79.3906667 70.264444 15.057778         NA 105.0115556        NA 77.2866667 68.343926 12.1348258      aa
  43 2015-03-09 72.0408889 59.5875556 68.546222 12.160444         NA  84.2777778        NA 70.9191111 61.255333 10.3376277      aa
  2  2014-06-19  0.0000000  0.0000000  0.000000  0.000000  0.0000000   0.0000000  0.000000  0.0000000  0.000000  0.0000000      ci
  8  2014-08-04  2.6173333  2.3884444  1.532889  0.000000  0.8017778   0.0000000  0.000000  0.9026667  1.030389  0.3748546      ci
  14 2014-09-17  0.6782222  0.9875556  2.130222  2.372889  2.2395556   0.8448889  4.001333  0.1568889  1.676444  0.4408737      ci
  20 2014-10-16  7.8302222  0.9142222  1.550222  1.792444  1.8346667   0.0000000        NA  0.0000000  1.988825  1.0170247      ci
  26 2014-11-14  6.9600000  1.2528889  1.005778  6.876444  4.8715556   1.0506667        NA 10.3613333  4.625524  1.3864420      ci
  32 2014-12-12 13.7844444  1.9368889  2.971556 10.785778         NA   1.5231111        NA  5.1284444  6.021704  2.0815131      ci
  38 2015-02-12  3.4862222  3.9262222  1.613333 10.524444         NA   2.2493333        NA  3.6991111  4.249778  1.3078608      ci
  44 2015-03-09  5.1493333  5.1004444  6.648000 14.676889         NA   9.3968889        NA  9.7488889  8.453407  1.4912960      ci
  </code></pre>
  
  <p>I am using this code to produce a plot:</p>
  
  <pre><code>ggplot(aaci, aes(x=Date, y=Average, colour=Species)) +
    geom_point(aes(colour=Species), size=4) +
    geom_line(aes(colour=Species), size=1.3) + 
    geom_errorbar(aes(ymin=Average-SE, ymax=Average+SE),
                  size = .7,
                  width=8,  colour="black",                  
                  position = position_dodge(.9)) +
    scale_y_continuous(limits = c(0,100), breaks=seq(0,100,10)) +
    labs(x=("Date of Panel Observation"), y=("Mean Cover (%)")) +
    scale_colour_manual(values = colz, breaks = c("aa", "ci"),labels=c(expression(italic('Ascidiella aspersa')), expression(italic('Ciona intestinalis')))) +
    theme_bw() +
  theme(panel.background = element_rect(colour = "black"),
        axis.text=element_text(size=14),
        axis.title.x = element_text(vjust=-0.2),
        axis.title.y = element_text(vjust=+0.6),
        axis.title=element_text(size=16,face="bold"),
        legend.justification = c(1, 1), legend.position = c(1, 1),
        legend.title=element_text(size=14),
        legend.text=element_text(size=12))
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/9apCH.jpg" alt="enter image description here"></p>
  
  <p>I have been trying to alter the x axis tick intervals to every month, to read: July, August, September, October, November, December, January, February, March.  As you can see I have succesfully manually edited my y axis. I have read many posts on changing x axis ticks but I can't seem to find a solution to my particular problem. Any assistance would be greatly appreciated.</p>