title: Legend for marker size
body: <p>I'm trying to but legend for marker size in Sccater plot
  there is some plot to describe what I want ( See Prediction Cost on the right )</p>
  
  <p><a href="https://i.stack.imgur.com/qDGEu.jpg" rel="nofollow noreferrer">Plot With Legend for marker size </a> </p>
  
  <p>I did all what I want except show legend for the marker size 
  Is there any property can solve my issue </p>
  
  <p><a href="https://i.stack.imgur.com/mZia6.png" rel="nofollow noreferrer">My plot</a></p>
  
  <p>My Code </p>
  
  <pre><code>fig = go.Figure()    
  fig.add_trace( 
                  go.Scatter(
                              x=YearData,
                              y=RatingData,
                              name=Key,
                              text=textHover,
                              marker_size=SizeData,
                              hoverinfo= "text"
                            )
               )
  
  
  
  fig.update_traces(mode='markers', marker=dict(sizemode='area', sizeref=2.*65/(40.**2), line_width=2))
  
  fig.update_layout(  title='تقييم مستشفيات وزارة الصحة في مدينة الرياض',
                      xaxis=dict( title='عام التقييم', gridcolor='white', type='log', gridwidth=2, ),
                      yaxis=dict( title='التقييم من 5', gridcolor='white', gridwidth=2, ),
                      paper_bgcolor='rgb(243, 243, 243)',
                      plot_bgcolor='rgb(243, 243, 243)',)
  fig.show()
  </code></pre>
  
  <p>thanks :) </p>