The resource histogram shows the daily consumption of resources (or weekly, monthly, etc. as long as it’s greater than the schedule’s time unit) for each activity.
In this simple example, we know Activity A requires 15 units each day of a combination of resources – specifically, about 10 of those are carpenters and about 5 of those are widgets. We know this by measuring the height at which each resource coincides with the histogram plot ruler. This is what the histogram does – it makes it quick and easy to see graphically which resources have been assigned to which activities and how much are required each day.
In this example, the activity’s duration has been changed from 10 days to 5 days. Note that it still requires 10 carpenters per day even after the duration has changed, which means that it must have been assigned 10 carpenters per day. The amount of widgets, however, has increased from 5 to 10, which means that Activity A must have been assigned 50 widgets per duration.
The resource curves show the total cumulative consumption of resources for every activity. For Activity A, the total consumption is derived by multiplying the histogram (i.e. daily consumption) by the total duration of Activity A. From example 1 to example 2, you can see that the total consumption drops from 150 to 100 as the duration decreases. Add the total consumption for every activity together and the result is the baseline curve, colored green. The early-dates curve, colored blue, shows what the total consumption would look like if every activity were on its early dates. The late-dates curve, colored red, shows what the total consumption would look like if every activity were on its late dates.