diff --git a/frontend/src/apexcharts.ts b/frontend/src/apexcharts.ts
index 8a2e5d37..70fc83c8 100644
--- a/frontend/src/apexcharts.ts
+++ b/frontend/src/apexcharts.ts
@@ -104,7 +104,7 @@ const axisTooltip =
       const formatted = Array.isArray(measured)
         ? measured.map(format).join(" - ")
         : format(measured);
-      if (axis === "y" && point.link)
+      if (axis === "y" && point?.link)
         valueSpan.innerHTML = linkTooltipValue(formatted, point.link);
       else valueSpan.innerText = formatted;
       axisValue.appendChild(valueSpan);
@@ -141,7 +141,7 @@ const sqlpage_chart = (() => {
     ["gray", "#495057", "#adb5bd"],
     ["black", "#000000", "#000000"],
     ["white", "#ffffff", "#f8f9fa"],
-  ];
+  ] as const;
   const colorNames = new Map(
     tblrColors.flatMap(([name, dark, light]): [string, string][] => [
       [name, dark],
@@ -200,8 +200,10 @@ const sqlpage_chart = (() => {
 
   function build_sqlpage_chart(c: HTMLElement) {
     const [data_element] = c.getElementsByTagName("data");
+    const chartContainer = c.querySelector<HTMLElement>(".chart");
+    if (!data_element || !chartContainer)
+      throw new Error("Chart component is missing its data or container");
     const data = read_chart_data(data_element.textContent);
-    const chartContainer = c.querySelector(".chart") as HTMLElement;
     chartContainer.innerHTML = "";
     const is_timeseries = data.time;
     const chart_type =
@@ -260,7 +262,10 @@ const sqlpage_chart = (() => {
       : aligned_series;
     const colors = is_pie
       ? points.map(
-          ({ color }, i) => named_color(color) || palette[i % palette.length],
+          ({ color }, i) =>
+            named_color(color) ??
+            palette[i % palette.length] ??
+            tblrColors[0][isDarkTheme ? 1 : 2],
         )
       : palette;
 
diff --git a/frontend/src/chart_series.ts b/frontend/src/chart_series.ts
index 52e6d7fb..333484f6 100644
--- a/frontend/src/chart_series.ts
+++ b/frontend/src/chart_series.ts
@@ -52,14 +52,21 @@ export function xaxis_type_for(
  * ascending order where they diverge
  */
 export function merged_x_values(series: ChartSeries[]): XValue[] {
-  const unread = series.map(({ data }) => data.map(({ x }) => x));
+  const unread = series.map(({ data }) => {
+    const iterator = data.values();
+    return { iterator, next: iterator.next() };
+  });
   const merged = new Map<PlotValue, XValue>();
-  while (unread.some((xs) => xs.length > 0)) {
-    const with_lowest_x = unread
-      .filter((xs) => xs.length > 0)
-      .reduce((a, b) => (is_lower(b[0], a[0]) ? b : a));
-    const x = with_lowest_x.shift() as XValue;
-    merged.set(x_key(x), x);
+  while (true) {
+    let lowest: { stream: (typeof unread)[number]; x: XValue } | undefined;
+    for (const stream of unread) {
+      if (stream.next.done) continue;
+      const x = stream.next.value.x;
+      if (!lowest || is_lower(x, lowest.x)) lowest = { stream, x };
+    }
+    if (!lowest) break;
+    merged.set(x_key(lowest.x), lowest.x);
+    lowest.stream.next = lowest.stream.iterator.next();
   }
   return [...merged.values()];
 }
diff --git a/frontend/src/sqlpage.ts b/frontend/src/sqlpage.ts
index efd31575..8570c3a3 100644
--- a/frontend/src/sqlpage.ts
+++ b/frontend/src/sqlpage.ts
@@ -103,10 +103,9 @@ function apply_number_formatting(table_el: HTMLElement) {
 
   for (const tr_el of table_el.querySelectorAll("tbody tr, tfoot tr")) {
     const cells = tr_el.getElementsByTagName("td");
-    for (let idx = 0; idx < cells.length; idx++) {
+    for (const [idx, cell_el] of [...cells].entries()) {
       const column_type = col_types[idx];
       const is_raw_number = col_rawnums[idx];
-      const cell_el = cells[idx];
       const text = cell_el.textContent;
 
       if (column_type === "number" && !is_raw_number && text) {
@@ -166,6 +165,8 @@ function setup_sort_behavior(
       items.sort((a, b) => {
         const a_key = a.sort_keys[button_index];
         const b_key = b.sort_keys[button_index];
+        if (!a_key || !b_key)
+          throw new Error("Table row is missing the selected column sort key");
         return (
           multiplier *
           (Number.isNaN(a_key.num) || Number.isNaN(b_key.num)
@@ -243,14 +244,21 @@ function sqlpage_map(root: InitRoot) {
     coords: string | undefined,
   ): Leaflet.LatLngTuple | undefined {
     if (!coords) return undefined;
-    const parsed = coords.split(",", 2).map((c) => Number.parseFloat(c));
-    if (parsed.length !== 2 || !parsed.every(Number.isFinite)) {
+    const [latitude, longitude] = coords
+      .split(",", 2)
+      .map((c) => Number.parseFloat(c));
+    if (
+      latitude === undefined ||
+      longitude === undefined ||
+      !Number.isFinite(latitude) ||
+      !Number.isFinite(longitude)
+    ) {
       console.error(
         `Invalid map coordinates: ${JSON.stringify(coords)}. Expected a "latitude,longitude" pair of numbers.`,
       );
       return undefined;
     }
-    return [parsed[0], parsed[1]];
+    return [latitude, longitude];
   }
   function onLeafletLoad(map_root: InitRoot) {
     is_leaflet_loaded = true;
diff --git a/tests/end-to-end/fixtures/chart/test.ts b/tests/end-to-end/fixtures/chart/test.ts
index 46635206..8038beb2 100644
--- a/tests/end-to-end/fixtures/chart/test.ts
+++ b/tests/end-to-end/fixtures/chart/test.ts
@@ -1,3 +1,4 @@
+import assert from "node:assert/strict";
 import { type ConsoleMessage, expect, type Page, test } from "../../fixture.ts";
 
 const MARKS =
@@ -219,8 +220,11 @@ test("positions complete numeric bar series on an explicit numeric axis (#733)",
   expect(chart.axisLabels.map(({ text }) => Number(text))).toEqual(xs);
   expect(chart.dataLabels.map(Number)).toEqual([...xs, ...xs]);
   expect(chart.barGroups).toHaveLength(xs.length);
-  for (const [index, label] of chart.axisLabels.entries())
-    expect(Math.abs(label.center - chart.barGroups[index])).toBeLessThan(1);
+  for (const [index, label] of chart.axisLabels.entries()) {
+    const barCenter = chart.barGroups[index];
+    assert.ok(barCenter !== undefined, `Bar group ${index} was drawn`);
+    expect(Math.abs(label.center - barCenter)).toBeLessThan(1);
+  }
 });
 
 test("keeps irregular numeric x values proportionately spaced", async ({
@@ -237,9 +241,9 @@ test("keeps irregular numeric x values proportionately spaced", async ({
     "3.0",
   ]);
   expect(chart.axisLabels.map(({ text }) => text)).not.toContain("2");
-  expect(chart.barGroups[2] - chart.barGroups[1]).toBeGreaterThan(
-    5 * (chart.barGroups[1] - chart.barGroups[0]),
-  );
+  const [first, second, third] = chart.barGroups;
+  assert.ok(first !== undefined && second !== undefined && third !== undefined);
+  expect(third - second).toBeGreaterThan(5 * (second - first));
 });
 
 test("keeps an explicit x interval count", async ({ page }) => {
@@ -299,7 +303,7 @@ test("gives a stacked series a zero at every x it did not measure", async ({
 
   expect(chart.failures).toEqual([]);
   expect(chart.series.map((s) => s.name)).toEqual(["CPU", "GPU"]);
-  expect(chart.series[1].points).toEqual([
+  expect(chart.series[1]?.points).toEqual([
     ["2024-01-01T00:00:00.000Z", 0],
     ["2024-01-01T00:01:00.000Z", 50],
     ["2024-01-01T00:02:00.000Z", 50],
@@ -310,10 +314,13 @@ test("gives a stacked series a zero at every x it did not measure", async ({
 test("stacks a series above the one it shares an x with", async ({ page }) => {
   const chart = await renderChart(page, "stacked-time-series");
   const [cpu, gpu] = chart.drawnPerSeries;
+  assert.ok(cpu && gpu, "Both series were drawn");
 
   expect(gpu.heights).toHaveLength(4);
   expect(gpu.heights[0]).toBe(cpu.heights[0]);
-  expect(gpu.heights[1]).toBeLessThan(cpu.heights[1]);
+  const cpuHeight = cpu.heights[1];
+  assert.ok(cpuHeight !== undefined, "CPU has a second point");
+  expect(gpu.heights[1]).toBeLessThan(cpuHeight);
 });
 
 test("keeps a lone series in the order the query returned it (#930)", async ({
@@ -322,7 +329,7 @@ test("keeps a lone series in the order the query returned it (#930)", async ({
   const chart = await renderChart(page, "out-of-order");
 
   expect(chart.failures).toEqual([]);
-  expect(chart.series[0].points).toEqual([
+  expect(chart.series[0]?.points).toEqual([
     ["Q3", 3],
     ["Q1", 1],
     ["Q2", 2],
@@ -335,12 +342,12 @@ test("orders by name the categories two bar series do not share (#951)", async (
   const chart = await renderChart(page, "disjoint-categories");
 
   expect(chart.failures).toEqual([]);
-  expect(chart.series[0].points).toEqual([
+  expect(chart.series[0]?.points).toEqual([
     ["X1", 0],
     ["X2", 10],
     ["X3", 30],
   ]);
-  expect(chart.series[1].points).toEqual([
+  expect(chart.series[1]?.points).toEqual([
     ["X1", 25],
     ["X2", 20],
     ["X3", 0],
@@ -353,7 +360,7 @@ test("leaves the points of a chart that does not stack alone", async ({
   const chart = await renderChart(page, "unstacked-time-series");
 
   expect(chart.failures).toEqual([]);
-  expect(chart.series[1].points).toEqual([
+  expect(chart.series[1]?.points).toEqual([
     ["2024-01-01T00:01:00.000Z", 50],
     ["2024-01-01T00:02:00.000Z", 50],
     ["2024-01-01T00:03:00.000Z", 50],
@@ -364,7 +371,7 @@ test("stacks a bar series on the categories it skipped", async ({ page }) => {
   const chart = await renderChart(page, "stacked-categories");
 
   expect(chart.failures).toEqual([]);
-  expect(chart.series[1].points).toEqual([
+  expect(chart.series[1]?.points).toEqual([
     ["Q1", 0],
     ["Q2", 20],
     ["Q3", 30],
@@ -377,7 +384,7 @@ test("lines an unstacked series up with the categories it skipped", async ({
   const chart = await renderChart(page, "line-categories");
 
   expect(chart.failures).toEqual([]);
-  expect(chart.series[1].points).toEqual([
+  expect(chart.series[1]?.points).toEqual([
     ["Q1", null],
     ["Q2", 20],
     ["Q3", 30],
@@ -389,6 +396,7 @@ test("draws nothing where an unstacked series has no value", async ({
 }) => {
   const chart = await renderChart(page, "line-categories");
   const [a, b] = chart.drawnPerSeries;
+  assert.ok(a && b, "Both series were drawn");
 
   expect(a.lefts).toHaveLength(3);
   expect(b.lefts).toEqual(a.lefts.slice(1));
@@ -397,8 +405,9 @@ test("draws nothing where an unstacked series has no value", async ({
 test("keeps a measured zero apart from a missing value", async ({ page }) => {
   const chart = await renderChart(page, "zero-and-missing");
   const [a, b] = chart.drawnPerSeries;
+  assert.ok(a && b, "Both series were drawn");
 
-  expect(chart.series[1].points).toEqual([
+  expect(chart.series[1]?.points).toEqual([
     ["Q1", null],
     ["Q2", 0],
     ["Q3", 30],
@@ -413,7 +422,11 @@ for (const type of ["area", "scatter", "heatmap"]) {
     const chart = await renderChart(page, `${type}-categories`);
 
     expect(chart.failures).toEqual([]);
-    expect(chart.series[1].points.map((p) => p[0])).toEqual(["Q1", "Q2", "Q3"]);
+    expect(chart.series[1]?.points.map((p) => p[0])).toEqual([
+      "Q1",
+      "Q2",
+      "Q3",
+    ]);
   });
 }
 
@@ -431,7 +444,7 @@ test("keeps the bubble size of the points it lined up", async ({ page }) => {
   const chart = await renderChart(page, "bubble-categories");
 
   expect(chart.failures).toEqual([]);
-  expect(chart.series[1].points).toEqual([
+  expect(chart.series[1]?.points).toEqual([
     ["Q1", null],
     ["Q2", 5],
   ]);
diff --git a/tests/end-to-end/official-site.spec.ts b/tests/end-to-end/official-site.spec.ts
index 8b1a483b..b008787a 100644
--- a/tests/end-to-end/official-site.spec.ts
+++ b/tests/end-to-end/official-site.spec.ts
@@ -77,7 +77,7 @@ test("stacked chart raises a series only where it has a value", async ({
   const gpu = await drawnPoints(powerChart, "GPU");
 
   expect([gpu[0], gpu[3]]).toEqual([cpu[0], cpu[3]]);
-  expect(Number(gpu[1].y)).toBeLessThan(Number(cpu[1].y));
+  expect(Number(gpu[1]?.y)).toBeLessThan(Number(cpu[1]?.y));
 });
 
 test("chart draws a yline as a line and a yline_end as a band", async ({
diff --git a/tests/js/chart_series.spec.ts b/tests/js/chart_series.spec.ts
index ed56fd22..34df1572 100644
--- a/tests/js/chart_series.spec.ts
+++ b/tests/js/chart_series.spec.ts
@@ -91,6 +91,20 @@ test("merged_x_values ignores series that hold no points", () => {
   assert.deepEqual(merged, [1]);
 });
 
+test("merged_x_values returns no labels when every series is empty", () => {
+  assert.deepEqual(merged_x_values([]), []);
+  assert.deepEqual(merged_x_values([series("a"), series("b")]), []);
+});
+
+test("merged_x_values keeps null and zero as distinct labels", () => {
+  const merged = merged_x_values([
+    series("a", { x: null, y: 1 }, { x: 1, y: 2 }),
+    series("b", { x: 0, y: 3 }),
+  ]);
+
+  assert.deepEqual(merged, [null, 0, 1]);
+});
+
 test("align_series gives every series a point at every x (#727)", () => {
   const [a, b] = align_series(
     [
@@ -100,6 +114,8 @@ test("align_series gives every series a point at every x (#727)", () => {
     ADDS_NOTHING_TO_THE_STACK,
   );
 
+  assert.ok(a);
+  assert.ok(b);
   assert.deepEqual(xs(a), ["Q1", "Q2"]);
   assert.deepEqual(xs(b), ["Q1", "Q2"]);
 });
@@ -113,7 +129,7 @@ test("align_series counts an x a stacked series skipped as zero (#727)", () => {
     ADDS_NOTHING_TO_THE_STACK,
   );
 
-  assert.deepEqual(b.data, [
+  assert.deepEqual(b?.data, [
     { x: "Q1", y: 0 },
     { x: "Q2", y: 3 },
   ]);
@@ -128,7 +144,7 @@ test("align_series leaves a gap where an unstacked series has no value", () => {
     LEAVES_A_GAP,
   );
 
-  assert.deepEqual(b.data, [
+  assert.deepEqual(b?.data, [
     { x: "Q1", y: null },
     { x: "Q2", y: 3 },
   ]);
@@ -143,7 +159,7 @@ test("align_series keeps a measured zero apart from a missing value", () => {
     LEAVES_A_GAP,
   );
 
-  assert.deepEqual(b.data, [
+  assert.deepEqual(b?.data, [
     { x: "Q1", y: null },
     { x: "Q2", y: 0 },
   ]);
@@ -155,7 +171,7 @@ test("align_series counts a null value as missing", () => {
     ADDS_NOTHING_TO_THE_STACK,
   );
 
-  assert.deepEqual(b.data, [{ x: "Q1", y: 0 }]);
+  assert.deepEqual(b?.data, [{ x: "Q1", y: 0 }]);
 });
 
 test("align_series keeps a blank value the series wrote", () => {
@@ -164,7 +180,7 @@ test("align_series keeps a blank value the series wrote", () => {
     ADDS_NOTHING_TO_THE_STACK,
   );
 
-  assert.deepEqual(b.data, [{ x: "Q1", y: "" }]);
+  assert.deepEqual(b?.data, [{ x: "Q1", y: "" }]);
 });
 
 test("align_series keeps a value the series wrote as text", () => {
@@ -173,7 +189,7 @@ test("align_series keeps a value the series wrote as text", () => {
     ADDS_NOTHING_TO_THE_STACK,
   );
 
-  assert.deepEqual(b.data, [{ x: "Q1", y: "7" }]);
+  assert.deepEqual(b?.data, [{ x: "Q1", y: "7" }]);
 });
 
 test("align_series keeps the third dimension of points it did not fill in", () => {
@@ -182,7 +198,7 @@ test("align_series keeps the third dimension of points it did not fill in", () =
     LEAVES_A_GAP,
   );
 
-  assert.equal(a.data[0].z, 42);
+  assert.equal(a?.data[0]?.z, 42);
 });
 
 test("align_series keeps the color of a point on the x it belongs to", () => {
@@ -195,7 +211,7 @@ test("align_series keeps the color of a point on the x it belongs to", () => {
   );
 
   assert.deepEqual(
-    b.data.map((point) => point.fillColor),
+    b?.data.map((point) => point.fillColor),
     [undefined, "#37b24d"],
   );
 });
@@ -209,9 +225,9 @@ test("align_series matches dates by value rather than by identity", () => {
     ADDS_NOTHING_TO_THE_STACK,
   );
 
-  assert.equal(a.data.length, 1);
-  assert.equal(b.data.length, 1);
-  assert.equal(b.data[0].y, 2);
+  assert.equal(a?.data.length, 1);
+  assert.equal(b?.data.length, 1);
+  assert.equal(b?.data[0]?.y, 2);
 });
 
 test("align_series leaves a lone series in the order it arrived (#930)", () => {
@@ -220,7 +236,7 @@ test("align_series leaves a lone series in the order it arrived (#930)", () => {
     LEAVES_A_GAP,
   );
 
-  assert.deepEqual(only.data, [
+  assert.deepEqual(only?.data, [
     { x: "Q2", y: 1 },
     { x: "Q1", y: 2 },
   ]);
@@ -256,7 +272,7 @@ test("align_series keeps the last of duplicated x values", () => {
     ADDS_NOTHING_TO_THE_STACK,
   );
 
-  assert.deepEqual(a.data, [
+  assert.deepEqual(a?.data, [
     { x: "Q1", y: 9 },
     { x: "Q2", y: 0 },
   ]);
@@ -269,7 +285,7 @@ test("align_series_for gives a stacked series a zero at every x it skipped", ()
     STACKED,
   );
 
-  assert.deepEqual(b.data, [
+  assert.deepEqual(b?.data, [
     { x: 1, y: 0 },
     { x: 2, y: 3 },
   ]);
@@ -286,7 +302,7 @@ for (const type of ["line", "area", "scatter", "bubble", "heatmap"]) {
       UNSTACKED,
     );
 
-    assert.deepEqual(b.data, [
+    assert.deepEqual(b?.data, [
       { x: "Q1", y: null },
       { x: "Q2", y: 3 },
     ]);
@@ -303,7 +319,7 @@ test("align_series_for counts a label a bar series skipped as zero", () => {
     UNSTACKED,
   );
 
-  assert.deepEqual(b.data, [
+  assert.deepEqual(b?.data, [
     { x: "Q1", y: 0 },
     { x: "Q2", y: 3 },
   ]);
diff --git a/tests/js/component_documentation.spec.ts b/tests/js/component_documentation.spec.ts
index c5dbc4ea..a9426cbe 100644
--- a/tests/js/component_documentation.spec.ts
+++ b/tests/js/component_documentation.spec.ts
@@ -144,6 +144,7 @@ function properties_of_template(source: string): TemplateProperties {
   const contexts: Context[] = ["top"];
 
   for (const [, raw_body] of source.matchAll(/\{\{([^}]*)\}\}/g)) {
+    assert.ok(raw_body !== undefined);
     let body = raw_body
       .replace(/^[{~]+/, "")
       .replace(/[~}]+$/, "")
@@ -203,7 +204,7 @@ function properties_of_template(source: string): TemplateProperties {
         continue;
       if (name.startsWith("this.")) name = name.slice(5);
       if (name === "this" || name === "") continue;
-      name = name.split(/[.[]/)[0]; // `a.b` and `a.[0]` are reads of `a`
+      name = name.split(/[.[]/)[0] ?? ""; // `a.b` and `a.[0]` are reads of `a`
       if (!/^[A-Za-z_][A-Za-z_0-9]*$/.test(name)) continue;
       if (LITERALS.has(name)) continue;
       if (parents === 0 && in_callee_position && HELPERS.has(name)) continue;
diff --git a/tsconfig.json b/tsconfig.json
index aa8793a1..eef2ff1b 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -10,6 +10,7 @@
     "isolatedModules": true,
     "verbatimModuleSyntax": true,
     "strict": true,
+    "noUncheckedIndexedAccess": true,
     "noImplicitOverride": true,
     "noFallthroughCasesInSwitch": true,
     "noUnusedLocals": true,