Can't set ChartViewBase's data property back to nil to clear out data

[ospr]

ChartViewBase's _data property is defined as an implicitly unwrapped optional when really it should be a plain optional. The value starts off as nil, but then once it has been set you can no longer set it back to nil to clear out the data in the graph. By making the _data property optional that will allow for clearing the chart data (which will allow the noDataText to be shown again) and will make use of _data safer because its not implicitly unwrapped (eg could cause a runtime error if the property is nil and is accessed).

Right now this code exists to prevent you from setting the _data property back to nil.

 if newValue == nil
 {
      print("Charts: data argument is nil on setData()", terminator: "\n")
      return
}

Note that if _data is changed from data! to data? that we'll need to handle all of the optional cases properly throughout the code base.