More work on ChartProxyModel. Now most of the functionality that's needed is implemented. Changes now allow us to set
* the data direction ( setDataDirection( Qt::Orientation ) and dataDirection() )
* whether the first column or row is to be treated as a header:
- setFirstRowIsHeader( bool ) and firstRowIsHeader( bool )
- setFirstColumnIsHeader( bool ) and firstColumnIsHeader( bool )

Also, mapToSource( QModelIndex ) works, and mapFromSource( QModelIndex ) needs some testing as I haven't done that yet.

With this implementation of the proxy model, it's expecting pure table data.
Headers in the source model - that is, accessible through sourceModel()->headerData() -
will be ignored.