he "where" bitmask from the list_chunks RPC --
// HACK -- it's a bitmask, but we treat it here as
// a discrete value.
var colormap = { 1:   "forestgreen",
                 2:   "steelblue",
                 4:   "steelblue",
                 8:   "steelblue",
                 16:  "steelblue",
                 32:  "gray",
                 64:  "gray",
                 128: "gray",
                 256: "cornflowerblue" };

var heightmap = {
    // RPC queued write request
    256: 0,
    // Downstream
   