[chicks-net]

Summary

Emphasize Y-axis gridlines at tick values that are multiples of 20 (major/minor grid pattern) using the existing theme color — no new query params. Implements chicks-net/chicks-projects#129.

Changes

• src/GraphCards.ts — add and export a Chartist plugin (majorGridPlugin) that listens for draw events and adds ct-grid-major to gridlines whose tick value data.axis.ticks[data.index] is numeric and % 20 === 0; wire it into getOptions() via plugins: [majorGridPlugin]. Chartist 0.11.4 (bundled via node-chartist) initializes plugins before creating the chart, so the listener catches every gridline. X-axis day ticks are strings, so the numeric check skips them. The tick-0 baseline darkens too (0 % 20 === 0), which reads as an intentional solid floor.
• src/styles/graphStyle.ts — add .ct-grid.ct-grid-major { stroke-opacity: 0.85; stroke-dasharray: none; } after the .ct-grid rule. The doubled-class selector out-specifies .ct-grid; inline SVG attributes would lose to the existing CSS rule, so a class is required.
• __test__/fakeInputs.ts — reuse majorGridPlugin in the test options so the utils.test.ts snapshot exercises the plugin (tick-0 gridline gains the class).
• Snapshots — regenerated utils.test.ts.snap (tick-0 gridline gains ct-grid-major) and svgs.test.ts.snap (new CSS rule in the graphSvg snapshots; tick-0 gridline gains the class in the chart snapshot).

Verification

• Full suite green locally: 13/13 tests, 4/4 snapshots; prettier/eslint clean on changed files.
• Local smoke test against node dist/main.js:
  • ashutosh00710 (window max < 20): only the zero baseline darkens, as predicted in the issue.
  • sindresorhus (step-50 scale: ticks 0/50/100/…/400): exactly 5 major lines (0, 100, 200, 300, 400) — the irregularity accepted in the issue's known-limitations section.

Deferred (per issue)

• Forcing ticks to multiples of 20 via Chartist.FixedScaleAxis
• A grid_major_color query param (reuses the existing theme color)
• Excluding the zero baseline from darkening (&& value > 0)

Checklist

• Followed the contributing guidelines
• Checked for other open PRs with the same change
• Submission passes tests (npm test: 13/13)
• Linted code locally (prettier + eslint clean on changed files)
• Explanation of changes and why included
• Tests updated as applicable (snapshot coverage for the new class)
• Successfully ran tests with changes locally

Refs chicks-net/chicks-projects#129 (auto-close won't fire across owners — close manually after live verification)