title: Set transparency (alpha) of area chart openpyxl
body: <p>I want to set transparency to the background of an area chart using openpyxl. The code I have for the chart is:</p>
  
  <pre><code>from openpyxl.drawing.fill import PatternFillProperties, ColorChoice
  
  c2 = AreaChart()
  c2.grouping = 'stacked'
  c2.add_data(data, titles_from_data=True)
  
  c2_s1 = c2.series[0]
  fill = PatternFillProperties()
  fill.background = ColorChoice(srgbClr="FFFFFF")
  
  c2_s2.graphicalProperties.pattFill = fill
  </code></pre>
  
  <p>I have tried using rgba hexadecimal encoding.</p>
  
  <pre><code>fill.background = ColorChoice(srgbClr="80FFFFFF")
  </code></pre>
  
  <p>I have also try to use <code>SystemColor</code> but I could not find any example of how to use it and every attempt I made was unsuccessful</p>