title: Manipulating axis titles in ggpairs (GGally)
body: <p>I'm using the code below to generate the following chart.</p>
  
  <pre><code># Setup
  data(airquality)
  
  # Device start
  png(filename = "example.png", units = "cm", width = 20, height = 14, res = 300)
  
  # Define chart
  pairs.chrt &lt;- ggpairs(airquality,
                        lower = list(continuous = "smooth"),
                        diag = list(continuous = "blank"),
                        upper = list(continuous = "blank")) +
    theme(legend.position = "none",
          panel.grid.major = element_blank(),
          axis.ticks = element_blank(),
          axis.title.x = element_text(angle = 180, vjust = 1, color = "black"),
          panel.border = element_rect(fill = NA))
  
  # Device off and print
  print(pairs.chrt)
  dev.off()
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/iAy3Z.png" alt="ggpairs - First Example"></p>
  
  <p>I'm currently trying to modify the display of the <strong>axis titles</strong>. In particular, I would like for the axis titles to be:</p>
  
  <ol>
  <li>Placed at a further distance from axis labels</li>
  <li>Placed at an angle</li>
  </ol>
  
  <p>As an example, I would like to obtain axis <strong>titles</strong> similar to the ones pictured below (I'm interested in axis labels only, not in rest of the chart):
  <img src="https://i.stack.imgur.com/MvSjV.png" alt="Example Label Placement">
  <em>Taken from : <a href="http://geovisualist.com/" rel="noreferrer" title="geovisualist">Geovisualist</a></em></p>
  
  <p>I' tried adjusting my syntax changing the <code>axis.title.x</code> to different values but it does not yield the desired results. For instance running the code with <code>angle = 45</code>.</p>
  
  <pre><code>axis.title.x = element_text(angle = 45, vjust = 1, color = "black"),
              panel.border = element_rect(fill = NA))
  </code></pre>
  
  <p>returns the same chart. I was able to control the axis labels by changing the <code>axis.text.x</code> for instance but I can't find the answer how to control the axis titles in this plot. Any help will be much appreciated.</p>