I have implemented a front-end for cell-centred Cactus AMR data. However, many Cactus simulations use vertex-centred AMR. That is, data is stored at the vertices of a grid (not in the cells), and coarse vertices are at the same locations as fine vertices.

How do I present such data to yt? Do I need to set grid coordinates differently, or is there a "vertex centred" flag?

When vertex-centred data are displayed, then I expect either a "control volume" around each vertex to have the same value, or to use e.g. linear interpolation in the cells from the respective boundaries.