Dataskera

Control Chart

A statistical process control chart for individuals: the centre line is the mean, the control limits sit at a chosen number of sigma, and sigma is estimated from the moving range so a drift does not widen its own limits. The Nelson rules do the watching, and every point they flag says which rule flagged it.

Data Requirements

Your Tableau worksheet should include:

  • 1 Dimension for the Sequence encoding. The order the measurements were taken in: a date, a batch, a sample number.
  • 1 Measure for the Value encoding. The measure being monitored.

Add Control Chart to your worksheet

First, download the manifest file. It tells Tableau where to load the extension from.

Download controlchart.trex

Direct link: https://dataskera.com/downloads/controlchart.trex

Then, in Tableau:

  1. On the Marks card, open the Mark type dropdown, and under Viz Extensions choose Add Extension.
  2. In the Add an Extension dialog, choose Access Local Viz Extensions, then open the controlchart.trex file you just downloaded.
  3. When Tableau asks, allow the extension to access the data in the workbook.
  4. The welcome screen appears. Sign in with your Dataskera account, or on Tableau Desktop choose Use on Desktop without an account. See Signing in & access below.
  5. Build the chart: drag the sequence dimension onto Sequence and the monitored measure onto Value.
Tableau Server or Cloud:
A site admin must allow the extension by adding its URL to the site's safe list, and the view needs the Download Summary Data permission. Extension URL: https://vizextension.dataskera.com/controlchart/controlchart.html

Signing in & access

Control Chart is free. Whether you need to sign in depends only on what you are doing. Viewing a dashboard never asks for an account, and authoring is free too.

Viewing a dashboard
No account

People who only view a dashboard are never asked to sign in and are never blocked.

Editing on Tableau Desktop
Optional

Sign in, or continue without an account in a single click.

Editing on Server or Cloud
Free account

Sign in with your Dataskera account to author on the web.

No account yet? Create one free in under a minute. No credit card required.

Format panel

Open the Format panel from the extension menu to customize the chart. It is for the dashboard author and is not shown to viewers. It has four tabs:

  • Control Limits: sigma from the moving range or the standard deviation, how many sigma the limits sit at, the limit labels, the one and two sigma zones, background.
  • Signals: which rules apply: a point beyond the limits, a run on one side of the centre line, a steady rise or fall, and the sigma zone rules, each with its own length where it has one.
  • Series: line width and point size.
  • Axis and Labels: the value axis and gridlines, value labels, how the sequence labels are angled, number format and font.
  • Color: a palette, a precise colour per category, and the overall opacity.
  • Legend: show or hide it, its position around the chart, an optional show/hide toggle for viewers, the colour swatch, and the content of each entry.
  • Tooltip: show or hide it, its colours, decimals, and a template you write yourself from the placeholders the chart provides.
A flagged point names its rule:
Beyond the limits, seven in a row above the mean, six rising, two of three past two sigma: the tooltip says which one fired. A point marked without a reason sends a quality engineer back to the raw data, which is the job the chart was supposed to do.
Moving range is the default for a reason:
Estimating sigma from the moving range of consecutive points means a shift in the process does not inflate the limits meant to catch it. The standard deviation is there when you want it.
No variation, no limits:
When every point carries the same value there is nothing to put limits around, and the chart says that rather than drawing three lines on top of each other.
A limit below zero is declared:
On a measure that never goes negative the lower limit can land below zero. That is correct arithmetic and an unreachable limit, so the chart says so instead of leaving an unexplained empty band under the series.