What is a Resource Histogram?
A resource histogram is a bar chart of resource demand over time — hours or headcount per week, stacked by role. Its diagnostic power is instant: peaks above capacity mean the plan is impossible as drawn; deep valleys mean paid idleness.
It's the picture that motivates leveling and smoothing: you level the peaks that break capacity, smooth the noise within float, and re-draw until the profile is one the organization can actually staff.
Worked example
The histogram for a refinery shutdown shows demand for certified welders spiking to 60 in week 3 against a market supply of 35. Discovered now, the fix is re-sequencing two exchanger bundles and pre-fabricating spools off-site. Discovered in week 3, the fix would have been overtime, day rates, and a blown window.