title: ggExtra plot format: similar marginal plots for different plot dimensions
body: <p>This question in regarding formatting plots produced using ggplot2 + ggExtra and is not related to any bug.</p>
  
  <pre><code>require(ggplot2)
  #&gt; Loading required package: ggplot2
  require(ggExtra)
  #&gt; Loading required package: ggExtra
  p1 &lt;- ggplot(data = mpg,aes(x = cty,y = cty)) + 
    geom_point()+ 
    xlab("City driving (miles/gallon)") +
    ylab("City driving (miles/gallon)")
  
  ggMarginal(p = p1,type= "boxplot")
  </code></pre>
  
  <p>The y-axis marginal plot in this chart is usually not similar to the x-axis marginal plot i.e. the width of the 2 boxplots are not similar. This problem become more acute when I change plot dimensions (in my case, using RStudio). Any suggestions how to make the width of the 2 boxplots similar while using different plot dimensions (width x height).</p>
  
  <p>I face similar problems with other marginal plot type options provided by ggExtra package: histogram, density. </p>