A Shewhart control chart plots measurements against run order with a centre line at the mean and warning (±2σ) and action (±3σ) limits.
Control charts are the foundation of statistical process control. The centre line is the in-control mean; the ±2σ warning limits flag points that warrant attention (about 5% expected by chance); the ±3σ action limits flag points where the process is almost certainly out of control (~0.3% expected by chance).
Western Electric rules extend the simple ±3σ test with patterns (8 points on the same side, 2 of 3 beyond ±2σ, etc.) — not implemented here, but the chart shows enough that you can spot them visually.
One numeric measurement + optional run-order column.
Line chart with centre + warning + action limits + count of out-of-control points.
A single point outside ±3σ is usually enough to investigate. A run of 7+ points on one side of the centre line or a monotone trend of 7+ points are early-warning patterns that suggest a slow drift even when no single point exceeds the action limits.