In a Grafana panel, how do thresholds, value mappings, units and field overrides relate to one another, and what is the relationship between a panel threshold and an alert?

## Two layers: defaults and overrides Every Grafana panel has a field configuration that is applied *after* transformations and *before* rendering. It has two layers. **Defaults** apply to every field in every frame the panel received: unit, decimal places, display name template, Min/Max, "No value" text, colour scheme, thresholds, value mappings, links, and panel-specific properties such as line width or fill opacity. **Overrides** are exceptions. Each override is a *matcher* plus a list of *properties*. Matchers include: fields with a specific name, fields matching a regular expression, fields of a given type, and fields returned by a specific query (query A vs query B). Properties are the same catalogue as the defaults. Overrides win over defaults, and later overrides win over earlier ones for the same property. This is how you put one series on a right-hand axis in percent while the rest stay on the left in bytes, without touching the query. The practical consequence: if a display setting looks stuck, check the override list before the defaults — a regex override you forgot is the usual culprit. ## Units, decimals and Min/Max Units are more than cosmetic. Grafana formats values by unit family (bytes, bytes/sec, seconds, percent (0–100) vs percent unit (0–1), currency), and picking the wrong one produces confidently wrong dashboards — the classic being *percent* against a ratio that is actually 0–1, giving a value of 0.97% instead of 97%. Decimals default to automatic. Min/Max bound the axis or gauge scale and, crucially, define what "percentage thresholds" mean. ## Thresholds A threshold configuration is a **base** colour plus ordered **steps**, each a value and a colour. A value takes the colour of the highest step it reaches or exceeds. Two modes exist: - **Absolute**: the step value is in the field's own units (latency > 0.5 s). - **Percentage**: the step is a percentage of the range between Min and Max. Without explicit Min/Max, that range is derived from the data and moves as the data moves, so percentage thresholds silently rescale. Set Min/Max whenever you use percentage mode. Where thresholds actually show up depends on the panel and the colour scheme. Set the colour scheme to *From thresholds* and stat/gauge/bar-gauge/table cells colour themselves. In a time series panel, thresholds can additionally be drawn as horizontal lines or shaded regions — a visual reference line, nothing more. Thresholds can also be made data-driven with the *Config from query results* transformation, which reads a per-row limit from a query and applies it as that field's threshold — useful when the limit is a property of the entity (per-service SLO) rather than a constant. ## Value mappings Value mappings translate raw values into human display text and optional colour. Four kinds: - **Value**: exact match (`0` → `DOWN` red, `1` → `UP` green). The standard fix for boolean-ish gauges. - **Range**: from/to, inclusive, for banding continuous values. - **Regex**: pattern on string values, with capture-group substitution for rewriting long identifiers. - **Special**: `null`, `NaN`, `null + NaN`, empty string, `true`/`false` — how you turn a gap into the word "No data" instead of a blank tile. Mappings are evaluated in order and the first match wins, so put the specific rules above the broad ones. Like everything else in field config, a mapping changes only what is drawn: the underlying numbers in the panel inspector are untouched, and a mapped label does not travel anywhere else. ## The threshold-versus-alert boundary This is the question inside the question. A panel threshold is display state, evaluated in the browser of whoever is looking at the dashboard. If nobody has the dashboard open, nothing happens. Alerting is a separate subsystem: rules are their own objects with their own queries, their own evaluation interval, their own pending/firing state machine and their own notification routing, evaluated by the Grafana server independent of any browser session. Never answer "I set a threshold on the panel" to "how would you get paged". The corollary matters operationally: because the two are separate, they drift. A panel coloured red at 500 ms while the rule fires at 800 ms will make every responder distrust the dashboard. Keeping the visual threshold and the alert condition numerically aligned — and reviewing them together when either changes — is the discipline that keeps a dashboard usable during an incident.