title: How to only change parameters for "lower" plots in the ggpairs function from GGally package
body: <p>I have the following example</p>
  
  <pre><code>data(diamonds, package="ggplot2")
  diamonds.samp &lt;- diamonds[sample(1:dim(diamonds)[1],200),]
  
  ggpairs(diamonds.samp, columns=8:10,
      upper=list(continuous='cor'),
      lower=list(continuous = 'points'),
      diag=list(continuous='density'),
      axisLabels='show'
  )
  </code></pre>
  
  <p>Resulting in a really nice figure:<img src="https://i.stack.imgur.com/yuVLW.png" alt="Result of ggpair function"></p>
  
  <p>But my problem is that in the real dataset I have to many points whereby I would like to change the parameters for the point geom. I want to reduce the dot size and use a lower alpha value. I can however not doe this with the "param" option it applies to all plot - not just the lower one:</p>
  
  <pre><code>ggpairs(diamonds.samp, columns=8:10,
          upper=list(continuous='cor'),
          lower=list(continuous = 'points'),
          diag=list(continuous='density'),
          params=c(alpha=1/10),
          axisLabels='show'
  )
  </code></pre>
  
  <p>resulting in this plot:
  <img src="https://i.stack.imgur.com/ukZEs.png" alt="Result of ggpair function"></p>
  
  <p>Is there a way to apply parameters to only "lower" plots - or do I have to use the ability to create custom plots as suggested in the topic <a href="https://stackoverflow.com/questions/11724351/how-to-adjust-figure-settings-in-plotmatrix/11732345#11732345">How to adjust figure settings in plotmatrix?</a>  </p>
  
  <p>In advance - thanks!</p>