Hello Gary ,
I am asking around here at Tableau on this one. Have you tried the free map service from Mapbox? I ask because that is what I use to get the sort of custom backgrounds that are described here.
Let us know where you are with this and if help is still needed.
Hi Patrick, thanks for responding back. Mapbox is actually what we ended up using. The only problem with Mapbox backgrounds is that you can't have the entire state context being displayed like you can with the image. I would love to know what's going on, as the Tableau documentation specifically mentions that you can use an image as a map background, although I can't remember if I've ever seen an example with an image behind a filled map. Mapbox works, but it seems like you should be able to use an image too.
Anything you hear back would be welcome as I'm sure this will come up again...
I've looked around a bit at your twbx and I think I understand *why* the problem is happening - with the image map you can only specify a single x and y coordinate per polygon (each polygon being a row in the table) so Tableau just drops each Geometry at the centroid (where the generated lat/lon points would be rendered if you were using a point map instead of a filled map). Since there isn't anything that specifies to Tableau how the polygons rendered by the Geometry are to be scaled they seem to just be defaulting to a small size.
A work around that you could try is to use the old pre-shapefile support hack to render the polygons: Grow your own Filled Maps
Essentially, take your geojson file, break each polygon into the individual vertices defining the polygon, and render the polygons using those coordinates (instead of as one single geometry for each polygon). That should make it so that each vertex is placed in the proper location and scaled to line up with your background map. I *think* that this should work...
You may want to tune the map view and the dashboard a little bit
to enable Zip Code Boundaries (and zoom to make them visible).
Please find the attached.