Enjoyed this, and the solve went fairly smoothly, but my resultant lines have left me with a few small doubts. The regions make sense broadly, but the topography doesn't actually match up particularly closely to the reality, as far as I can tell. Does this seem as expected?
Also, if one follows the preamble strictly, particularly with respect to the line being "through the centres of the cells," technically speaking, the number of "distinct regions" created doesn't actually match the three specified. I get the intention but given the specific language in the preamble, as well as my above concern, I wasn't certain.