title: ggpairs plot with heatmap of correlation values
body: <p>My question is twofold;</p>
  
  <p>I have a ggpairs plot with the default <code>upper = list(continuous = cor)</code> and I would like to colour the tiles by correlation values (exactly like what ggcorr does).</p>
  
  <p>I have this: <img src="https://i.stack.imgur.com/PCWK0.jpg" alt="ggpairs plot of daily flows"><br>
  I would like the correlation values of the plot above to be coloured like this: <img src="https://i.stack.imgur.com/WShvC.jpg" alt="ggcorr heatmap of correlation values"></p>
  
  <pre><code>library(GGally)
  
  sample_df &lt;- data.frame(replicate(7,sample(0:5000,100)))
  colnames(sample_df) &lt;- c("KUM", "MHP", "WEB", "OSH", "JAC", "WSW", "gaugings")
  
  ggpairs(sample_df, lower = list(continuous = "smooth"))  
  ggcorr(sample_df, label = TRUE, label_round = 2)
  </code></pre>
  
  <p>I had a brief go at trying to use <code>upper = list(continuous = wrap(ggcorr)</code> but didn't have any luck and, given that both functions return plot calls, I don't think that's the right path?</p>
  
  <p>I am aware that I could build this in ggplot (e.g. <a href="https://stackoverflow.com/questions/29075305/what%C2%B4s-the-best-way-to-do-a-correlation-matrix-plot-like-this">Sandy Muspratt's solution</a>) but given that the GGally package already has the functionality I am looking for I thought I might be overlooking something.</p>
  
  <hr>
  
  <p>More broadly, I would like to know how we, or if we can, call the correlation values? A simpler option may be to colour the labels rather than the tile (i.e. <a href="https://stackoverflow.com/questions/21716567/use-ggpairs-to-create-this-plot">this question</a> using colour rather than size) but I need a variable to assign to colour... </p>
  
  <p>Being able to call the correlation values to use in other plots would be handy although I suppose I could just recalculate them myself.</p>
  
  <p>Thank you!</p>