What is a Cumulative Flow Diagram (CFD)?
A cumulative flow diagram stacks the count of work items in each state (to do, in progress, done) over time as colored bands. Read it like an X-ray of your process: the vertical distance through the middle bands is WIP, the horizontal distance is approximate lead time, and the top band's slope is arrival rate versus the done band's slope, throughput.
A widening middle band is the diagram screaming: work is entering faster than it's leaving. Flat "done" slope, growing WIP — the bottleneck is visible before anyone feels it.
Worked example
A platform team's CFD shows the "code review" band tripling in width over three weeks while "done" flattens — two seniors who did most reviews went on leave, and nobody noticed the queue forming. The CFD did, a fortnight before the sprint goals started missing. WIP limit on review + review rotation, and the bands parallel again.