library(ggplot2)
library(pROC)

# mROC is a ROC object
myauc <- paste("AUC=",round(auc(mROC), digits=2), sep='')
p <- ggroc(mROC) + annotate(geom="text", x=0.8, y=0.85, label=myauc) + 
          theme(panel.grid.major = element_line(color = '#e0e0e0', size=0.2), 
          panel.grid.minor = element_blank(), 
          panel.background = element_blank(), 
          axis.line = element_line(colour = "black")
          )

library(ggplot2)
library(pROC)

# svmROC, gbmROC, rfROC: ROC object
ggroc(list(svm=svmROC, gbm=gbmROC, randomForest=rfROC)) + 
theme_bw() + 
theme(legend.title = element_blank(), 
      legend.position = c(0.8, 0.2),
      legend.text = element_text(size=12),
      text = element_text(size=12)) + 
      scale_color_manual(values=c("#000000", "#0072B2", "#E69F00", "#CC79A7"), 
                       labels=c(paste("gbm: ", round(gbmROC$auc,4), sep=""), 
                                paste("randomForest: ", round(rfROC$auc, 4), sep=""), 
                                paste("svm: ", round(svmROC$auc, 4), spe=""),
                                paste("nnet: ", round(nnetROC$auc, 4), sep="")
                                )
                       ) +
    labs(title="", x="Specificity", y="Sensitivity")