title: Adding row names on y-axis in geom_tile plot
body: <p>I am plotting a data frame as a heatmap (using geom_tile) and I want the row names from my input matrix to be along the y-axis. Here is my code:</p>
  
  <pre><code>head(data)
                   0h_rep2   0h_rep1   1h_rep2   1h_rep1   4h_rep2   4h_rep1   9h_rep2  9h_rep1    15h_rep2  15h_rep1  18h_rep2  18h_rep1  21h_rep2  21h_rep1
  hsa-let-7d-5p  11.485307 11.660549 11.383846 11.623624 11.395039 11.499499 11.660549 11.515537   11.697380 11.794755 11.803184 11.899935 12.225001 12.006919
  hsa-let-7e-5p   10.660692 10.732285 10.869931 10.966696 10.984550 10.942785 10.966696 10.853484  10.963071 11.072640 11.315740 11.290014 11.643941 11.360874
  
  pd &lt;- as.data.frame(scale(t(data)))
  pd$Time &lt;- sub("", "", rownames(pd))
  pd.m &lt;- melt(pd)
  
  head(pd.m)
   Time      variable      value
  1 0h_rep2       11     -0.8495269
  2 0h_rep1       11     -0.1199689
  3 1h_rep2       11     -1.2719237
  
  pd.m$variable &lt;- as.numeric(factor(pd.m$variable, levels =     rev(as.character(unique(pd.m$variable))), ordered=F))
  p &lt;- ggplot(pd.m, aes(Time, variable))
  p  + geom_tile(aes(fill = value)) + 
   scale_fill_gradient2(low=muted("blue"), high=muted("red")) +
   scale_x_discrete(labels=c("0h_rep2", "0h_rep1","1h_rep2","1h_rep1","4h_rep2","4h_rep1","9h_rep2","9h_rep1", "15h_rep2", "15h_rep1","18h_rep2","18h_rep1","21h_rep2","21h_rep1")) + 
   theme_bw(base_size=20) + 
   theme(axis.text.x=element_text(angle=0, vjust=0.5, hjust=0, size=12),    axis.text.y=element_text(size=12), strip.text.y=element_text(angle=0, vjust=0.5, hjust=0.5, size=12),
                               strip.text.x=element_text(size=12)) +     labs(y="Genes", x="Time (h)", fill="")
  </code></pre>