title: Pick functionality from one usercontrol to another
body: <p>I have dialog which consist of two user controls bind to their models.</p>
  
  <p><img src="https://i.stack.imgur.com/9pK3H.png" alt="Dialog with user controls"></p>
  
  <p>I want to enable user to pick a data point on the chart control from the UserControl1.</p>
  
  <p>I can get the data point on mouse down event of chart control and wan to fill the associated textbox with that value. I am unable to identify the best approach to pass that value back to the UserControl1. Also how will I identify which button has made the call to pick the data point. Note I have to do the same in an MVVM friendly manner.</p>