Waterfall Chart ignores field_display_names and leaks internal __wf_lead field into axis title

Summary

On Waterfall charts, field_display_names is ignored and the internal window-transform field __wf_lead leaks into the x-axis title. Line/Bar/Area charts apply field_display_names correctly, so this is Waterfall-template–specific.

Verified against flint-chart@0.3.0 (via flint-chart-mcp@0.3.0). Source refs at main (commit 95b2552).

Repro

compile_chart, backend vegalite:

{
  "chart_spec": {
    "chartType": "Waterfall Chart",
    "encodings": { "x": { "field": "week" }, "y": { "field": "dau_change" } }
  },
  "field_display_names": { "dau_change": "DAU weekly change", "week": "Week (Mon, JST)" }
}

Observed: compiled spec has "title": "dau_change" on the y encoding; the x-axis title renders as week, __wf_lead. Neither display name is applied.

Expected: y-axis title "DAU weekly change", x-axis title "Week (Mon, JST)", no internal field in any title.

The same input on Line / Bar / Area charts applies both display names correctly — so the generic assembler path is fine; only the Waterfall template is affected.

Root cause

packages/flint-js/src/vegalite/templates/waterfall.ts, instantiate():

1. buildVLEncodings (in vegalite/assemble.ts) already writes the display name onto resolvedEncodings.{x,y}.title. The template destructures ctx.resolvedEncodings but discards those titles, hardcoding the raw field name:

   const { x, y, color, column, row } = ctx.resolvedEncodings;
   // ...
   y: {
       field: "__wf_prev_sum",
       type: "quantitative",
       title: yField,          // ← raw field, ignores y.title (the display name)
       ...
   },
   

2. The shared xEnc sets no title, so VL auto-derives it from the field. The connector-rule layer adds x2: { field: "__wf_lead" } (also untitled), and VL concatenates every untitled field on the shared x scale into one axis title → "week, __wf_lead".

Suggested fix

Use the titles the assembler already resolved, and suppress internal fields:

const xTitle = x?.title ?? xField;
const yTitle = y?.title ?? yField;

const xEnc = {
    field: xField,
    type: "ordinal" as const,
    sort: null,
    axis: { labelAngle: -45 },
    title: xTitle,                      // was absent
};

// bar layer:
y: { field: "__wf_prev_sum", type: "quantitative", title: yTitle, /* ... */ },

// connector layer — internal fields must never title the shared axis:
x:  { field: xField,      type: "ordinal", sort: null, bandPosition: 0, title: null },
x2: { field: "__wf_lead", bandPosition: 1, title: null },

General guard: any layer encoding bound to a __wf_* / internal field should set title: null (or axis: { title: null }) so VL never surfaces it.

Start in packages/flint-js/src/vegalite/templates/waterfall.ts, especially instantiate(), then compare its resolved encodings with buildVLEncodings in vegalite/assemble.ts. Reproduce the compile_chart example with the Vega-Lite backend and verify that the configured x and y display names appear while __wf_lead does not contribute to any axis title.