I think fogmachine has the right idea.
In CAD we have this problem, which is addressed by layer config settings which govern the order of display, with the last one drawn displaying on top with the others showing through below (transparency). There are various ways of handling this type of priority and transparency if it is not bitmapped, but vector data.
I believe Weatherfax_pi is using bitmapped images
, so if it is made to be the last to display (on top) the other layers may not show through. I believe grib images
are vector data (which would have transparency), but I may have that wrong.