You'll notice a strange discontinuity in the yellow and black circles. That's where you reach passive unhittability, even when shield block is down. We can interpret this as follows: When you are still hittable, the damage reduction given by Hold the Line is, relatively, not as important as keeping your dodge and parry balanced and covering the combat table. But once you never take an unmitigated hit, then increasing parry and dodge are only pushing blocks off the combat table as opposed to full hits. At this point, Hold the Line becomes more prominent and we should favor parry more heavily.
While I was doing this, I examined the relative damage reduction given by parry, dodge, mastery and armor. I found something interesting, though I suppose not unexpected. We have known for a while that when we are hittable, mastery is our best statistic. It covers the combat table faster than any other stat. Furthermore, due to the increasing returns of the critical block mechanic and diminishing returns on parry and dodge, there is a breakpoint (which we passed in Tier 11) such that a point of mastery also reduces more damage (over a long period of time) than a point of parry or dodge would.
However, once we are passively unhittable, parry and dodge become superior under the measure of long-run damage taken. When mastery is only increasing our critical block, it seems better to reforge it back into parry and dodge.
But this is only if we are trying to maximize damage reduction at all other costs. When we become passively unhittable, then parry, dodge, and mastery all yield a fraction of the returns they currently do. This is due to steep diminishing returns on parry and dodge, and the fact that mastery would only increase critical blocks. When we reach passive unhittability, the best strategy may be to start gemming and enchanting for increased stamina as our damage reduction is, roughly speaking, about as good as it can get.