u Chart in Excel
The purpose of a u chart is to determine stability of "counted" data (e.g., errors per bill, dents in a car door, etc.) when there can be more than one defect per unit and the sample size varies.
u chart data looks like this:
Example of a u chart created using the QI Macros SPC Software for Excel.
The stair step or skyline control limits are the result of the varying sample sizes.