title: How to plot across several layout regions
body: <p>There are times when I am composing a multi-part figure, and I have a complex layout, and I would like to plot outside of the box.  (I say this figuratively and literally).</p>
  
  <p>Consider this example using the <code>R</code> base graphics function <code>layout()</code> to set a complex layout:</p>
  
  <pre><code>## Define the layout regions
  multiPartFigureLayout &lt;- structure(c(4, 4, 1, 1, 2, 2, 3, 3, 8, 8, 
                                           5, 5, 1, 1, 2, 2, 3, 3, 8, 8, 
                                           9, 9, 1, 1, 2, 2, 3, 3, 8, 8, 
                                           6, 6, 1, 1, 2, 2, 3, 3, 8, 8, 
                                           7, 7, 1, 1, 2, 2, 3, 3, 8, 8), 
                                           .Dim = c(10L, 5L))
  
  ## Demonstrate the layout
  win.graph(4, 5)
  layout(multiPartFigureLayout)
  layout.show(9)
  </code></pre>
  
  <p>This produces the following plotting layout.  (I have added text in red using image editing software)</p>
  
  <p><img src="https://i.stack.imgur.com/U7h0l.jpg" alt="enter image description here"></p>
  
  <p>This is one application of plotting outside the box: overprinting text in the area shown.  Equally, it might be necessary to overprint another graphical element.  For example, to draw a rule that spans the boxes.</p>
  
  <p>I am aware that it is possible to expand the <code>multiPartFigureLayout</code> matrix and add a plotting region above that could hold the text or the rule.  But I don't want to do this:  I want to overprint it across multiple layout regions.</p>
  
  <p>Is there a way to use base graphics to do this, either using a feature for this purpose, or by tricking the plotting functions in some way?</p>