title: How set ticks x axis with datetime on ggplot
body: <p>I'm plotting a dataset a little over three months long with a data each half hour.
  So in my dataframe I have a datetime vector with the format <code>%Y-%m-%d %H:%M:%OS</code> - year, month, day, hour, minute, second.
  I want just to set the ticks on the x-axis each fiveteen days.
  I tried with <code>scale_x_date</code> and <code>scale_x_datetime</code> setting the <code>breaks</code>, but without result and an error message.
  I didn't understand well the difference between <code>scale_x_date</code>, <code>scale_x_datetime</code>, but I think that in my case, with a datetime vector I should use <code>scale_x_datetime</code>.</p>
  
  <p>It's very diffcult share my dataset, because is very large and using a smaller portion I thing wouldn't give enough material to figure out my problem.</p>
  
  <p>So I share the code that I used and the graphical output that I have.</p>
  
  <pre class="lang-r prettyprint-override"><code>library(ggplot2) 
  
  ggplot(fluxmxrdesp,aes(x=Datetime))+
    geom_line(aes(y=mz31_mxr,colour="Formaldehyde"), size = 1)+
    labs(y = "VMR (ppb)" , x = "Date",colour="Legend")+
    scale_colour_manual(name='',values = ("Formaldehyde"="darkolivegreen4"))+
    theme_classic()+theme(legend.position = "top") + 
    theme(axis.title.y = element_text(margin = margin(r=25),color="black", size=18),
          axis.text.y = element_text(color="black",size=14),
          axis.title.x = element_text(margin = margin(t=10),color="black", size=18),
          axis.text.x = element_text(color="black", size=14),
          legend.text = element_text(color = "black", size = 18))
  </code></pre>
  
  <p>Short dataset</p>
  
  <pre><code>structure(list(Datetime = structure(c(1527813000, 1527814800, 
  1527816600, 1527818400, 1527820200, 1527822000), class = c("POSIXct", 
  "POSIXt"), tzone = "%Y-%m-%d %H:%M:%OS"), H = c(-6.088316625, 
  -9.882886968, -6.533935906, -5.305478189, -3.844176459, -12.90132526
  ), LE = c(0.946172041403071, -1.10377384331459, -1.53126820765533, 
  -2.28713506105331, 1.59264111550727, -1.08866725213062), Turbulence = c(0.0838, 
  0.128583643, 0.0806, 0.0875, 0.0733, 0.168911167), Wind_dir = c(255.3307133, 
  277.4016533, 281.2972942, 274.410293, 241.1132665, 256.6879056
  ), mz31_flux = c(0.00754, 0.0393, -0.00867, -0.0075, -0.00622, 
  0.0347), mz33_flux = c(-0.0898, 0.162, -0.0944, 0.0466, 0.0614, 
  -0.271), mz39_flux = c(0.131, -0.245, -0.157, 0.153, 0.147, 0.322
  ), mz42_flux = c(-0.00644, 0.00684, 0.0103, 0.0135, -0.0039, 
  0.017), mz45_flux = c(0.0245, 0.107, 0.031, 0.025, -0.0294, -0.0719
  ), mz47_flux = c(-0.031, -0.061, -0.0345, 0.0482, -0.0244, 0.0803
  ), mz59_flux = c(0.0309, 0.0639, 0.0345, 0.0424, 0.0231, -0.0926
  ), mz61_flux = c(-0.013, -0.0187, -0.0155, 0.0382, 0.00854, 0.053
  ), mz69_flux = c(0.0208, 0.0213, -0.00855, 0.011, -0.0112, -0.0213
  ), mz71_flux = c(0.00807, -0.00698, 0.00235, 0.00435, -0.00648, 
  0.0109), mz75_flux = c(-1.83e-17, -0.00405, -0.000647, 0.000867, 
  -0.000523, 0.0108), mz79_flux = c(-0.00999, 0.03, 0.00945, 0.00893, 
  0.0136, -0.028), mz85_flux = c(0.00681, -0.0219, 0.00518, -0.0091, 
  -0.00905, -0.0183), mz87_flux = c(0.00908, 0.0112, 0.00519, 0.00973, 
  0.00647, -0.0135), mz93_flux = c(-0.0299, 0.0532, 0.0417, 0.0203, 
  -0.0189, 0.0637), mz99_flux = c(-0.00727, 0.00771, 0.00569, -0.0047, 
  -0.00118, -0.00661), mz101_flux = c(0.00489, 0.0114, 0.00326, 
  -0.00676, 0.0051, 0.0165), mz107_flux = c(-0.0279, -0.0268, 0.0211, 
  0.0167, -0.0084, 0.0372), mz111_flux = c(-0.00163, 0.00664, -0.00186, 
  0.00456, 0.00155, 0.00682), mz113_flux = c(-0.0013, 0.0056, 0.000907, 
  0.00154, -0.00125, 0.00679), mz135_flux = c(-0.00619, -0.00983, 
  0.00429, 0.00534, 0.00503, 0.0231), mz137_flux = c(0.0233, 0.0346, 
  0.0171, -0.000554, 0.0282, -0.0985), mz149_flux = c(-0.00176, 
  -0.00336, 0.000902, -0.000605, -0.000848, 0.00901), mz155_flux = c(0.00116, 
  -0.00137, 0.000528, 0.000541, -5.79e-05, -7.08e-05), mz31_mxr = c(7.27e-06, 
  0.000172, 7.29e-06, 7.69e-06, 6.69e-06, 0.000154), mz33_mxr = c(0.0042, 
  0.00439, 0.00349, 0.00321, 0.00334, 0.00323), mz39_mxr = c(0.0038, 
  0.0038, 0.00364, 0.00352, 0.0033, 0.00323), mz42_mxr = c(0.000102, 
  0.000135, 7.58e-05, 0.000112, 9.04e-05, 0.000143), mz45_mxr = c(0.00128, 
  0.0012, 0.00107, 0.001, 0.00101, 0.00099), mz47_mxr = c(0.000273, 
  0.000476, 0.000236, 0.000398, 0.000264, 0.000476), mz59_mxr = c(0.00205, 
  0.00194, 0.00193, 0.0019, 0.00182, 0.00172), mz61_mxr = c(3.78e-05, 
  0.000182, 0.000124, 0.000261, 0.000156, 0.000241), mz69_mxr = c(0.000299, 
  0.000313, 0.000291, 0.000255, 0.00026, 0.000252), mz71_mxr = c(7.59e-05, 
  6.53e-05, 6.75e-05, 6.59e-05, 6.21e-05, 6.33e-05), mz75_mxr = c(2.21e-21, 
  1.42e-05, 1.89e-07, 3.56e-07, 3.07e-07, 2.16e-05), mz79_mxr = c(0.000346, 
  0.000406, 0.000382, 0.00031, 0.000383, 0.000293), mz85_mxr = c(0.000151, 
  0.00014, 0.000143, 0.000133, 0.000123, 0.000111), mz87_mxr = c(9.9e-05, 
  9.82e-05, 9.68e-05, 8.91e-05, 8.39e-05, 8.74e-05), mz93_mxr = c(0.00228, 
  0.00268, 0.00252, 0.00208, 0.0024, 0.00189), mz99_mxr = c(3.69e-05, 
  6.19e-05, 3.81e-05, 5.54e-05, 1.45e-05, 4.66e-05), mz101_mxr = c(0.000155, 
  0.000155, 0.000157, 0.000147, 0.000137, 0.000139), mz107_mxr = c(0.00129, 
  0.00152, 0.00142, 0.00115, 0.00133, 0.00104), mz111_mxr = c(1.09e-05, 
  5.27e-05, 9.35e-06, 5.86e-06, 6.03e-06, 3.74e-05), mz113_mxr = c(2.15e-05, 
  3.78e-05, 2.07e-05, 2e-05, 1.89e-05, 3.51e-05), mz135_mxr = c(0.000153, 
  0.000141, 0.000115, 9.73e-05, 9.94e-05, 8.94e-05), mz137_mxr = c(0.000142, 
  0.000122, 4.16e-05, 1.78e-06, 7.94e-05, 0.000197), mz149_mxr = c(1.58e-06, 
  2.22e-05, 1.13e-06, 6.74e-07, 9.28e-07, 1.84e-05), mz155_mxr = c(1.57e-06, 
  4.71e-06, 3.58e-07, 4.42e-07, 4.64e-08, 4.9e-08)), row.names = c(NA, 
  6L), class = "data.frame")
  </code></pre>
  
  <p>As you can see in the figure (<a href="https://i.stack.imgur.com/C1sjh.png" rel="nofollow noreferrer">here</a>) the tick, as text, for each month.
  I'd like to have something like: <code>01/06/2018 00:00, 15/06/2018 00:00, 1/07/2018 00:00</code> and so on.
  Also with the possibility of reducing the window between one tick and another</p>