title: Error bars on stacked barchart, using either plotly or ggplotly
body: <p>I need to convert into widget a simple ggplot, a stacked bar with uncertainty. </p>
  
  <p>The data:</p>
  
  <pre><code> world.tot &lt;- data.frame('country'='world', 'GHG'=c('CH4', 'CO2','N2O'), 
                           'emi'=c(6e+6, 3e+6, 1+6),
                           'unc.min'=8561406, 'unc.max'=14027350)
  </code></pre>
  
  <p>and the ggplot: </p>
  
  <pre><code>p2 &lt;- ggplot(world.tot) +
          geom_bar(aes(x=country,y=emi,fill=GHG), stat='identity', position='stack' ) +
          geom_errorbar(aes(x=country, ymin=unc.min, ymax=unc.max), width=0.2) +
          theme(axis.title. x=element_blank(), axis.title. y=element_blank()) +
          theme(legend.position='none')
  </code></pre>
  
  <p>When I try: <code>ggplotly(p2)</code> only the stacked bars are converted, not the error bar. Any advice?</p>
  
  <p>Alternatively, I could use <code>plot_ly</code> to create the plot, but cannot manage to add the error bar:</p>
  
  <pre><code>plot_ly(world.tot, x=~country. y=~emi, color=~GHG,type=bar,
          error_y=~list(array(c(unc.min, unc.max))) %&gt;% 
    layout(barmode='stack')
  </code></pre>
  
  <p>This produces error bars to all shares of the stacked histogram, while I need only one error at the top of the stacked histogram.</p>
  
  <p>Any help is appreciated</p>