title: Add common legend to multiple pROC plots
body: <p>I've plotted multiple ROC curves with <code>pROC</code> and now I want to add a common legend horizontally across the bottom-centre of the plot space. I typically use <code>ggplot</code> and now I'm lost with base R plotting.</p>
  
  <p>Here's my code:</p>
  
  <pre><code># Set plotting parameters
  par(mfrow=c(1,2), pty = "s", oma = c(4,1,1,1))
  
  # ROC plot 1
  roc(logit_A$y, logit_A$fitted.values, plot=TRUE, legacy.axes=FALSE, percent=TRUE, col="salmon", lwd=2, print.auc=TRUE)
  plot.roc(logit_B$y, logit_B$fitted.values, percent=TRUE, col="goldenrod", lwd=2, print.auc=TRUE, add=TRUE, print.auc.y=40)
  plot.roc(logit_C$y, logit_C$fitted.values, percent=TRUE, col="lightsteelblue", lwd=2, print.auc=TRUE, add=TRUE, print.auc.y=30)
  title(main = "Model 1", line = 2.5)
  
  # ROC plot 2
  roc(logit_A2$y, logit_A2$fitted.values, plot=TRUE, legacy.axes=FALSE, percent=TRUE, col="salmon", lwd=2, print.auc=TRUE)
  plot.roc(logit_B2$y, logit_B2$fitted.values, percent=TRUE, col="goldenrod", lwd=2, print.auc=TRUE, add=TRUE, print.auc.y=40)
  plot.roc(logit_C2$y, logit_C2$fitted.values, percent=TRUE, col="lightsteelblue", lwd=2, print.auc=TRUE, add=TRUE, print.auc.y=30)
  title(main = "Model 2", line = 2.5)
  
  # Add legend
  legend("bottom",
         legend=c("A", "B", "C"),
         col=c("salmon", "goldenrod", "lightsteelblue"),
         lwd=4, cex =0.4, xpd = TRUE, horiz = TRUE)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/M86ZK.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/M86ZK.png" alt="enter image description here"></a>
  How do I make it so the legend is centered along the bottom between both plots?</p>