[yt-users] Particle Density Projection for Flash Data

Dear all, I have just started using yt3 and would like to display particle densities in a projection plot. It seems that for Enzo data, I can use ('deposit', 'io_density'). With Flash data, however, dd['deposit', 'io_density'] will give me an error message: KeyError: "Unable to open object (Object 'particle_posy' doesn't exist)" I have checked and the particles are there. dd['io', 'particle_posy'] does return the particle y positions, so I am confused. Are there other ways of projecting particle densities for Flash data? Thank you! Yuan