title: plotly does not show titles of multiple plots in R
body: <p>I am trying to layout 2 plots together using <code>ggplot2</code> and <code>plotly</code>. Here's what I tried:</p>
  
  <pre><code>library(ggplot2)
  library(plotly)
  
  mt_mpg &lt;- ggplot(data = mtcars)+
    geom_boxplot(aes(x = as.factor(cyl), y = mpg))+
    ggtitle("mpg vs cyl")
  
  mt_disp &lt;- ggplot(data = mtcars)+
    geom_boxplot(aes(x = as.factor(cyl), y = disp))+
    ggtitle("disp vs cyl")
  
  subplot(mt_mpg, mt_disp)  
  </code></pre>
  
  <p>Everything works great but the title of the combined plot only contains "disp vs cyl". I want to include both titles on the top of their corresponding plots. But I don't see any option in <code>subplot()</code> command to do so. Any ideas how this can be fixed? Thanks.</p>