How can I define an overriding set of columns within a DataGrid-based control of Gantt Chart Light Library?
You need to define a full column collection as the Columns property value:
<pdgcc:GanttChartDataGrid.Columns>
<pdgcc:DataGridColumnCollection>
<hd:DataTreeGridColumn Header="Task"/>
<dataControls:DataGridTextColumn Header="Start" Binding="{Binding Start, Mode=TwoWay, Converter={StaticResource DateTimeStringConverter}}"/>
<dataControls:DataGridTextColumn Header="Finish" Binding="{Binding Finish, Mode=TwoWay, Converter={StaticResource DateTimeStringConverter}}"/>
</pdgcc:DataGridColumnCollection>
</pdgcc:GanttChartDataGrid.Columns>
Note that without adding the collection container tag, the specified columns would be just added to the existing collection (i.e. after the default, previously existing, columns).