MapProjectionDisplay question

I am trying to use the IDV classes to build navigated displays. I learnt the whole 'display side coordinate system' technique from in the visad/examples. In particular, how a call to MapProjection.setDefaultMapArea with a xy rectangle would result in the visad 'box' showing the xy area.

When moving to the higher level idv classes, I thought the corresponding call was in ucar.unidata.view.geoloc.MapProjectionDisplay.setMapArea ( ProjectionRect). This does not seem to do what I want. I did peek at the source code and noted that the comments for that method imply that the rectangle is in fact a lat/lon rectangle, not a cartesian one.

Can anyone shed any light on the setMapArea (and setMapRegion) methods, and how 
to use them correctly?