diff --git a/CHANGELOG.md b/CHANGELOG.md
index b7a5e5ea..54333135 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,7 @@
  - `column` charts now display vertical bars instead of nothing at all.
  - `stacked` is now ignored on chart types that cannot stack, instead of displaying an empty chart.
  - Screen readers now announce the title of the modal component instead of an unnamed dialog.
+ - Charts can display reference lines. A row with a `yline` is drawn as a line across the chart at that value of the y axis, with `yline_label` and `yline_color` for its text and its color. Reference lines are rows, so a chart can have as many of them as the query returns. A line follows its axis, so on a `horizontal` bar chart a `yline` is drawn down the chart rather than across it. They are not added to the total of a `stacked` chart, and are not filled in an `area` chart.
 
 ## v0.45
 
diff --git a/examples/official-site/sqlpage/migrations/01_documentation.sql b/examples/official-site/sqlpage/migrations/01_documentation.sql
index 9e4ce254..72e9eef5 100644
--- a/examples/official-site/sqlpage/migrations/01_documentation.sql
+++ b/examples/official-site/sqlpage/migrations/01_documentation.sql
@@ -675,7 +675,10 @@ INSERT INTO parameter(component, name, description, type, top_level, optional) S
     ('y', 'The value of the point on the vertical axis', 'REAL', FALSE, FALSE),
     ('label', 'An alias for parameter "x"', 'REAL', FALSE, TRUE),
     ('value', 'An alias for parameter "y"', 'REAL', FALSE, TRUE),
-    ('series', 'If multiple series are represented and share the same y-axis, this parameter can be used to distinguish between them.', 'TEXT', FALSE, TRUE)
+    ('series', 'If multiple series are represented and share the same y-axis, this parameter can be used to distinguish between them.', 'TEXT', FALSE, TRUE),
+    ('yline', 'Draws a reference line across the chart at this value of the y axis instead of plotting a point, to show a limit such as a quota or an alarm threshold. Not drawn if it falls outside of the axis, so set ymax when the limit is above the data.', 'REAL', FALSE, TRUE),
+    ('yline_label', 'A text to display next to the yline.', 'TEXT', FALSE, TRUE),
+    ('yline_color', 'The name of a color for the yline. Grey by default.', 'COLOR', FALSE, TRUE)
 ) x;
 INSERT INTO example(component, description, properties) VALUES
     ('chart', 'An area chart representing a time series, using the top-level property `time`.
@@ -780,6 +783,65 @@ The `color` property sets the color of each series separately, in order.
         {"series": "Yearly maintenance", "label": "Maintenance", "value": ["2022-01-01", "2022-01-03"]}
     ]')),
     ('chart', '
+## Reference lines
+
+A row with a `yline` is not plotted as a data point, but drawn as a line across
+the whole chart, at that value of the y axis. Use it for the limit that the data
+should be read against: a disk quota, an alarm threshold, a service level
+objective.
+
+Reference lines are rows, so they come from a query like everything else,
+and a chart can have as many of them as the query returns:
+
+```sql
+select ''chart'' as component, ''CPU temperature'' as title, true as time, 100 as ymax;
+select celsius as yline, name as yline_label, color as yline_color from thresholds;
+select measured_at as x, celsius as y from readings order by measured_at;
+```
+
+They are drawn as annotations rather than as an extra series, so they are not
+added to the total of a `stacked` chart, and are not filled in an `area` chart.
+
+A line outside of the y axis is not drawn, and does not stretch the axis to fit,
+so set `ymax` when the limit is above the data.
+', json('[
+        {"component":"chart", "title": "CPU temperature", "type": "line", "time": true,
+         "ytitle": "°C", "ymax": 100, "color": "azure", "marker": 4},
+        {"yline": 70, "yline_label": "target", "yline_color": "green"},
+        {"yline": 90, "yline_label": "throttling", "yline_color": "red"},
+        {"x": "2024-05-01T08:00:00Z", "y": 52},
+        {"x": "2024-05-01T09:00:00Z", "y": 58},
+        {"x": "2024-05-01T10:00:00Z", "y": 71},
+        {"x": "2024-05-01T11:00:00Z", "y": 83},
+        {"x": "2024-05-01T12:00:00Z", "y": 94},
+        {"x": "2024-05-01T13:00:00Z", "y": 76},
+        {"x": "2024-05-01T14:00:00Z", "y": 63}
+    ]')),
+    ('chart', '
+## Reference lines follow their axis
+
+A reference belongs to the column it is written in, not to a direction on the
+screen: `yline` always marks a value of `y`, whichever way round the chart is
+drawn. A `horizontal` bar chart runs its y axis from left to right, so a `yline`
+is drawn down the chart rather than across it.
+
+```sql
+select ''chart'' as component, ''bar'' as type, true as horizontal, 100 as ymax;
+select 90 as yline, ''full'' as yline_label, ''red'' as yline_color;
+select host as x, percent_used as y from disks order by percent_used;
+```
+
+A `pie` has no axes, and ignores reference lines.
+', json('[
+        {"component":"chart", "title": "Disk usage", "type": "bar", "horizontal": true,
+         "ymax": 100, "color": "azure", "labels": true},
+        {"yline": 90, "yline_label": "full", "yline_color": "red"},
+        {"x": "backup-1", "y": 41},
+        {"x": "web-2", "y": 63},
+        {"x": "db-1", "y": 88},
+        {"x": "web-1", "y": 96}
+    ]')),
+    ('chart', '
 ## Multiple charts on the same line
 
 You can create information-dense dashboards by using the [card component](?component=card#component)
diff --git a/sqlpage/apexcharts.js b/sqlpage/apexcharts.js
index cd1d1d58..b461fa31 100644
--- a/sqlpage/apexcharts.js
+++ b/sqlpage/apexcharts.js
@@ -116,6 +116,45 @@ sqlpage_chart = (() => {
   if (typeof module !== "undefined")
     module.exports = { align_series, align_series_for, merged_x_values };
 
+  const referenceColor = colorNames[isDarkTheme ? "gray-lt" : "gray"];
+
+  /** @typedef { {[property:string]: string|number|null} } ReferenceLine */
+
+  /** @param {string|number|null} name */
+  const reference_color = (name) =>
+    (typeof name === "string" && colorNames[name]) || referenceColor;
+
+  /**
+   * @param {ReferenceLine[]} rows - the rows that carry a yline
+   * @param {"x"|"y"} axis - the apexcharts axis the y column is drawn on
+   * @param {(value: any) => any} to_axis_value - puts a SQL value on the axis
+   * @returns {object[]} apexcharts axis annotations
+   */
+  function y_reference_lines(rows, axis, to_axis_value) {
+    return rows.flatMap((row) => {
+      if (row.yline == null) return [];
+      const from = to_axis_value(row.yline);
+      if (Number.isNaN(from)) return [];
+      const color = reference_color(row.yline_color);
+      const annotation = {
+        [axis]: from,
+        borderColor: color,
+        fillColor: color,
+        strokeDashArray: 4,
+      };
+      // apexcharts reads label.text unconditionally, so an annotation without
+      // a label must not have the key at all.
+      if (row.yline_label)
+        annotation.label = {
+          text: row.yline_label,
+          orientation: "horizontal",
+          borderColor: color,
+          style: { background: color, color: isDarkTheme ? "#000" : "#fff" },
+        };
+      return [annotation];
+    });
+  }
+
   /** @param {HTMLElement} c */
   function build_sqlpage_chart(c) {
     const [data_element] = c.getElementsByTagName("data");
@@ -127,9 +166,11 @@ sqlpage_chart = (() => {
       APEXCHARTS_TYPE_ALIASES[data.type] || data.type || "line";
     const is_stacked =
       !!data.stacked && STACKABLE_CHART_TYPES.includes(chart_type);
+    const points = data.points.filter(Array.isArray);
+    const reference_rows = data.points.filter((row) => !Array.isArray(row));
     /** @type { Series } */
     const series_map = {};
-    for (const [name, old_x, old_y, z] of data.points) {
+    for (const [name, old_x, old_y, z] of points) {
       series_map[name] = series_map[name] || { name, data: [] };
       let x = old_x;
       let y = old_y;
@@ -157,12 +198,27 @@ sqlpage_chart = (() => {
     let labels;
     const categories = x_is_text(series);
     if (chart_type === "pie") {
-      labels = data.points.map(([name, x, _y]) => x || name);
-      series = data.points.map(([_name, _x, y]) => Number.parseFloat(y));
+      labels = points.map(([name, x, _y]) => x || name);
+      series = points.map(([_name, _x, y]) => Number.parseFloat(y));
     } else if (series.length > 1)
       series = align_series_for(series, chart_type, is_stacked);
 
+    const to_value =
+      is_timeseries && chart_type === "rangeBar"
+        ? (v) =>
+            (typeof v === "number" ? new Date(v * 1000) : new Date(v)).getTime()
+        : Number;
+    const inverted =
+      chart_type === "rangeBar" || (chart_type === "bar" && !!data.horizontal);
+    const value_axis = inverted ? "x" : "y";
     const options = {
+      annotations: {
+        [`${value_axis}axis`]: y_reference_lines(
+          reference_rows,
+          value_axis,
+          to_value,
+        ),
+      },
       chart: {
         type: chart_type,
         fontFamily: "inherit",
diff --git a/sqlpage/templates/chart.handlebars b/sqlpage/templates/chart.handlebars
index 34d3256e..4621bdad 100644
--- a/sqlpage/templates/chart.handlebars
+++ b/sqlpage/templates/chart.handlebars
@@ -40,12 +40,19 @@
     "points": [
     {{~#each_row~}}
         {{~#if (gt @row_index 0)}},{{/if~}}
+        {{~#if yline~}}
+        {
+            "yline": {{~stringify yline}},
+            "yline_label": {{~stringify yline_label}}, "yline_color": {{~stringify yline_color}}
+        }
+        {{~else~}}
         [
             {{~ stringify (default series (default ../title "")) ~}},
             {{~ stringify (default x label) ~}},
             {{~ stringify (default y value) ~}}
             {{~#if z}}, {{~ stringify z ~}} {{~/if~}}
         ]
+        {{~/if~}}
     {{~/each_row~}}
     ]
 }
diff --git a/tests/end-to-end/official-site.spec.ts b/tests/end-to-end/official-site.spec.ts
index c81117c3..8afad3c2 100644
--- a/tests/end-to-end/official-site.spec.ts
+++ b/tests/end-to-end/official-site.spec.ts
@@ -78,6 +78,38 @@ test("stacked chart raises a series only where it has a value", async ({
   expect(Number(gpu[1].y)).toBeLessThan(Number(cpu[1].y));
 });
 
+test("chart draws a reference line for every yline", async ({ page }) => {
+  await page.goto(`${BASE}/documentation.sql?component=chart#component`);
+
+  const temperature = page.locator(".card", {
+    has: page.getByRole("heading", { name: "CPU temperature" }),
+  });
+  await expect(temperature.locator(".apexcharts-canvas")).toBeVisible();
+
+  const annotations = temperature.locator(".apexcharts-yaxis-annotations");
+
+  await expect(annotations.locator("line")).toHaveCount(2);
+  await expect(annotations.getByText("target")).toBeVisible();
+  await expect(annotations.getByText("throttling")).toBeVisible();
+});
+
+test("chart draws a yline down a horizontal chart", async ({ page }) => {
+  await page.goto(`${BASE}/documentation.sql?component=chart#component`);
+
+  const disks = page.locator(".card", {
+    has: page.getByRole("heading", { name: "Disk usage" }),
+  });
+  await expect(disks.locator(".apexcharts-canvas")).toBeVisible();
+
+  await expect(disks.locator(".apexcharts-xaxis-annotations line")).toHaveCount(
+    1,
+  );
+  await expect(disks.locator(".apexcharts-yaxis-annotations line")).toHaveCount(
+    0,
+  );
+  await expect(disks.getByText("full")).toBeVisible();
+});
+
 test("map", async ({ page }) => {
   await page.goto(`${BASE}/documentation.sql?component=map#component`);
   await expect(page.getByText("Loading...")).not.toBeVisible();