Size & scale

The gauge

One number, one needle, and two limits somebody typed in.

a.k.a. the dial · speedometer chart · the needle · radial gauge · the half-donut · the KPI dial · the meter · min and max · the barometer face · the fuel gauge

A gauge turns one number into an angle, and the angle is set by three things, only one of which is the number. There is no baseline and usually no zero: the needle lands at the value minus the minimum, over the maximum minus the minimum — the minimum shifting every reading and the width dividing it, two settings and neither of them the measurement, both typed in once by whoever installed the thing. Move either and the same measurement moves round the face. On top of that sit the coloured zones, which do something a chart normally leaves to the reader: they convert a continuous quantity into a verdict, at edges drawn long before the data arrived and identical in every context the gauge is ever hung in. And beneath both is the thing a gauge cannot do at all. It has one mark and no time axis, so it can only show a level — where the number is now. Any quantity whose meaning lives in its rate of change is invisible on one, and two moments that mean opposite things arrive as the same picture. That last failure is shared with every single-value display, a big number on a tile included; what a gauge adds is the other two, because it is the snapshot with a scale drawn round it and a verdict printed on the scale.

How to spot it

Find the minimum and the maximum, then ask who chose them. They are not measurements; they are settings, and together they decide how far round the needle goes.
Redraw it in your head with the range halved or doubled. If the reading moves from “nearly full” to “barely started”, the drama was in the limits.
Ask what value sits at the start of the scale. A gauge whose minimum is not zero is a truncated axis bent into a circle, with the truncation hidden by the curve — and unlike a bar chart it often has no baseline to restore, which makes declaring the range the whole of the fix.
Look for the coloured bands and ask where the edges came from — and note that they were drawn before the data arrived, which is the exact inverse of shopping for class breaks afterwards and no safer. An edge round in one unit and ragged in another (28.5 inches of mercury, 965.1 hectopascals) is a threshold inherited from an instrument rather than derived from anything, and a reading just inside one is reported exactly like a reading at the far end of it.
Ask what happens off the ends. A value past the maximum is drawn pinned at the maximum, which is the clipped axis arriving in a second body.
Look for the missing axis: time. There is one mark, so there is no yesterday on it, no rate, and no direction of travel. Ask whether the quantity is one people actually judge by its change.
Two gauges that look identical are the tell. Real measurements taken hours apart rarely tie, and a dial that cannot separate a crisis from its aftermath is answering a narrower question than the reader is asking.
The places they hide: dashboard tiles and BI “KPI” widgets, car and cockpit instruments, smart-home panels, capacity and utilisation meters, health-score rings, and the household barometer, which is the ancestor of all of them.

The fix

Give the number back its history. A single value almost never means anything on its own, so the default replacement for a gauge is a small line or a sparkline of the last however-many periods with the current reading printed beside it — same tile, same space, and now the reader can see the level, the direction and the rate at once. Where the rate is the finding, plot the rate: a change-over-the-last-interval chart answers the question the dial was being consulted for, and it is usually a number the system already has, since the sensor that reported the level generally reported the tendency in the same breath. Where a gauge really is right — a live instrument, glanced at, read for a threshold rather than an amount — treat its two limits as an axis and label them accordingly: put the range in the caption the way you print a unit, hold it fixed across every gauge on the page, run the minimum to zero wherever the quantity has one, and never let a value past the maximum be drawn pinned to it. Keep the zones for decisions that genuinely are categorical, say what each edge is and who set it, and print the number as well as the word, since a reading one unit inside a boundary and a reading at the far end of it are the same colour and not the same news. And for comparing several quantities, drop the form entirely. A row of gauges is a row of separate, unrelated scales, each with its own limits — which is the free-scale small multiple in circular dress. One sorted, zero-based bar chart puts them all on the same ruler and takes less room than the dials it replaces.

In the gallery

Dark navy screen laid out as a household wall display, masthead “Hall display · barometer · a cottage on Nantucket · Thu 4 Jan 2018 · 08:53 EST” on the left and “Source: KACK METAR · hourly” on the right, headlined “Barometer: RAIN.” A standfirst says this is one reading printed exactly as the station reports it — 989 hPa, sea-level pressure, from the automated weather station at the airport; that nothing is smoothed, averaged or rounded away; that the dial is not truncated but drawn across its full 950 to 1050 hPa in even 10-point steps with no break and no cap; that the words are the ones barometer faces have carried since long before the aneroid, STORMY, RAIN, CHANGE, FAIR, VERY DRY; and that the needle sits in RAIN, a little over a third of the way round. The read-out for the household is rain later, bring the cushions in, and take a coat to the 11:15 ferry. The main panel, headed “Sea-level pressure · now” and subtitled “Full dial, 950 to 1050 hPa · 270° sweep · five zones · nothing hidden, nothing capped”, is dominated by a large circular barometer face. Its outer band is divided into five zones, shaded from dark at the bottom of the scale to pale at the top and labelled STORMY, RAIN, CHANGE, FAIR and VERY DRY around the rim; ticks run every 5 hPa with numbers every 10 from 950 at the lower left round to 1050 at the lower right; and a single gold needle points just below 990, into the RAIN zone, with the word RAIN printed beside it. Under the dial the reading is set out as “989.1 hPa · 29.21 inHg” with a note that this is 39.1% of the way round the dial, 105.6° of the 270° sweep. Five tiles sit beside the dial. “The five words and where they start” tabulates each zone with its range in hectopascals and its lower edge in inches of mercury: STORMY up to 965.1, RAIN 965.1 to 999.0 from 28.50 inHg, CHANGE 999.0 to 1,015.9 from 29.50, FAIR 1,015.9 to 1,032.8 from 30.00, VERY DRY above 1,032.8 from 30.50, with the RAIN row picked out in gold. “Dial settings” gives minimum 950, maximum 1050, sweep 270°, major ticks every 10 hPa and minor every 5, zone edges at 28.5, 29.5, 30.0 and 30.5 inHg, set once on installation. “How to read the dial” says to find the needle and read the word behind it, that further round means higher pressure and better weather while further back means lower and worse, and that the figure under the dial is the pressure itself so nothing has to be estimated by eye. “Why the dial reads 989” says because that is what the station says. “Where the words come from” notes that the five-word face is older than the aneroid — it was on mercury wheel and banjo barometers long before Vidi’s 1844 movement inherited it — that it encodes a real piece of weather lore, low pressure going with unsettled air and high with settled, and that the zone edges are round numbers in inches of mercury because that is the unit the faces were engraved in. “No judgement required” notes the reading is reported to a tenth of a hectopascal and that at 989.1 the needle sits 9.9 hPa clear of the CHANGE edge above it and 24.0 hPa clear of the STORMY edge below — nowhere near a boundary. Eight cards run down the right. “The reading, in one figure” prints 989 hPa in large type. “The forecast, in one word” prints RAIN in large gold type, noting that no interpretation is required of the reader because the words are printed on the face. “Read-out for the household” repeats the cushions and the ferry and adds that the gutter can wait for the weekend. “The plan for today” lists the 11:15 ferry to Hyannis, cushions in off the deck, gutter at the weekend, and the Bakers arriving at four. “What the dial leaves out” says nothing, as far as the pressure goes — the whole scale is on the face, every zone is labelled, the needle is where the number puts it, no axis has been cut, no scale broken, no maximum capped and no colour ramp bent. “Why we like the dial” says you can read it from the stairs, and that it has been in the hall for eleven years without once being wrong about a Sunday. “And if the weather turns” says we shall see it out of the window. A “Basis of measurement” card credits sea-level pressure from station KACK, Nantucket Memorial Airport, from the hourly METAR of 4 January 2018 at 13:53 UTC, that is 08:53 EST — the group SLP891, or 989.1 hPa. A method line runs along the foot: one instantaneous reading, full 950–1050 hPa dial, 270° sweep, no truncation, no break, no cap, five zones at the traditional edges, reading printed to a tenth of a hectopascal. A footnote credits the Iowa Environmental Mesonet ASOS archive and says the display is a demonstration drawn by us in the manner of a household weather panel — the cottage, the household and the read-out are invented, and the figure behind them is real.

The needle read 989 twice, and the dial never said STORMY

The gauge The truncated axis