title: How to create a floating graph marker with ios-charts
body: <p>I'm using the ios-charts framework and want to create a marker that floats over the graph when I touch and move my finger from side to side.  I'm using a line chart just for reference.</p>
  
  <p>However, I can't figure out how to get the position of the marker on the chart when I touch it.</p>
  
  <p>I've read about subclassing MarkerView but that isn't working.  Can you please show me how to get the position of the point that shows up when you touch an ios-chart?</p>