Slowing down, or growing scope?
A burndown that flattens looks the same either way. The burnup separates them by drawing total scope as its own line — so this draws both.
Burndown and burnup from the same data
Both charts, because a burndown alone cannot tell you whether the team slowed down or the work grew.
Day by day
Burndown — what is left
Burnup — what is done, against total scope
How the lines are drawn
remaining = scope on that day − completed by that day
The ideal line runs from the scope at the start down to zero on the last day.
It is not rebased when scope changes. Redrawing the ideal line against today's larger scope makes a growing project look like it is tracking perfectly, which defeats the point of drawing it. Added work should make the chart look worse, because it is worse.
Completed work must be cumulative and non-decreasing. If it falls, that is a data error rather than negative progress, and it is refused rather than plotted.
Why a burndown alone misleads
A burndown that flattens has exactly two explanations and shows neither: the team stopped finishing things, or someone added work. Those call for completely different responses, and the chart most teams look at every day cannot distinguish them.
The burnup separates them by plotting total scope as its own line. Delivery is the rising line; scope is the ceiling. When the ceiling moves, you can see it moved, and the conversation becomes about the change rather than about whether the team is trying hard enough.
Scope reduction is not progress
The mirror image is worth naming. Removing work from a sprint makes a burndown drop, and it looks exactly like delivery. Nothing was built. On a burnup the scope line comes down to meet the delivery line, which is honest and considerably less flattering.
Neither chart is a management tool
Both are information radiators for the team doing the work. Used as a performance measure they stop being either honest or useful, because the easiest way to improve a burndown is to change what gets counted — and that will happen, quietly, within about two sprints of anyone being judged on it.