Make it easier to use the Chart Macro by deducing the data source type

Right now we always default to "xdom" (ie data source is a table defined in a document). However this is not very nice when using the chart macro with data defined in the macro content.

The new strategy to implement is:

• if the user has specified a data source type then use it
• Otherwise if the macro has content defined, default to the inline DS and if not then default to the "xdom" DS