Chart builder: discoverable entry points — Build Chart door, dataset picker, no-datasets state

src/core/chart-builder.ts

@@ -1034,6 +1034,30 @@ export function pruneEncodings(config: BuilderConfig, base: BuilderColumns): Bui
   return changed ? { ...config, encodings } : config;
 }
 
+/**
+ * Re-point an in-progress config at a different dataset (spec §06 → Dataset
+ * picker). Chart-level intent survives the switch — mark, title/subtitle,
+ * explicit size, sort/stack, calculated fields, and expression filters (their
+ * `datum` references are surfaced by the unknown-field feedback, not dropped) —
+ * while anything bound to a column the new dataset doesn't have is shed:
+ * encodings via `pruneEncodings`, predicate filters by field lookup. With a
+ * same-schema dataset (the common switch: a fresher version of the same data)
+ * everything survives verbatim.
+ */
+export function rebaseBuilderConfig(
+  config: BuilderConfig,
+  datasetName: string,
+  columns: BuilderColumns,
+): BuilderConfig {
+  const available = new Set(effectiveColumns(columns, config.calculates).columns);
+  const filters = (config.filters ?? []).filter(
+    (f) => f.mode === 'expression' || (f.field !== undefined && available.has(f.field)),
+  );
+  const rebased: BuilderConfig = { ...config, datasetName };
+  if (config.filters !== undefined) rebased.filters = filters;
+  return pruneEncodings(rebased, columns);
+}
+
 /** A built Vega-Lite spec, as a plain object (serialize with `buildSnippetSpecText`). */
 export type ChartSpec = Record<string, unknown>;