That does make sense; we're using that multi-color approach for the report in the example in https://secure.phabricator.com/T12403. But we also put labels per-category, which is possible in that report because the categories are continuous over time to form ribbons, unlike this chart. For this chart, I think the mental model is that all categories are really sub-categories of the parent category "stuff we have worked on", and users only need to distinguish between high-priority and low-priority, hence the shading.

@ablekh Thank you.

Here are some additional use cases and a sample implementation that is a Burn-up chart, not a Burndown chart, plus rationale.

Here is an example of a report that is satisfying several use cases from the comment above, https://secure.phabricator.com/T10339#159098. Those use cases, fleshed out a bit, include:

More use cases:

But I agree with Kevin that the point limit seems a lot less useful than the task limit. They're both useful for different reasons, but having been a scrum master and on agile teams for the past 10 years, I find the task limit more fundamental to the processes I've seen.