title: Plotly 'Download as png' resizing image when used in RShiny application
body: <p>I'm using <code>plotly</code> package for <code>R</code> to display some large graphs in my <code>shiny</code> application, the graphs display nicely as intended. However, when I click the "Download as png" button, the downloaded .png has been resized. <a href="https://michael-bird-94.shinyapps.io/rodeexplorer/" rel="noreferrer">here is a demo of this behavior</a>.</p>
  
  <p>How can I specify the resolution for the exported plot? </p>
  
  <p>Here is a minimal example that demonstates the issue by having a really long title that gets clipped on download</p>
  
  <p><strong>app.R</strong></p>
  
  <pre><code>library(shiny)
  library(ggplot2)
  library(plotly)
  
  ui &lt;- fluidPage(titlePanel("Plotly Download demo"),
  
                  plotlyOutput("demoPlotly"))
  
  
  server &lt;- function(input, output) {
    output$demoPlotly &lt;- renderPlotly({
      #make an arbritrary graph with a long title
      p &lt;- iris %&gt;%
        ggplot(aes(x = Petal.Length, y = Petal.Width, color = Species)) +
        geom_point() +
        labs(title = "This is a really long title to demo my problem of plotly resizing my downloaded output")
      ggplotly(p)
  
    })
  }
  
  # Run the application
  shinyApp(ui = ui, server = server)
  </code></pre>
  
  <p>The downloaded graph looks like this:
  <a href="https://i.stack.imgur.com/zA43U.png" rel="noreferrer"><img src="https://i.stack.imgur.com/zA43U.png" alt="Clipped plotly graph"></a></p>