/**
 * Histogram object from mcp-ui generate_histogram_chart.
 * yAxe is always empty. xAxe is the one binned column.
 * groupBy stays empty: a histogram of one variable is not a grouped chart,
 * and groupBy must not repeat xAxe.
 * data is one root whose histogramResults use _id as an object, never a string.
 */
export declare function histogramChartPayload(chartType: string | undefined, fields: Record<string, string>, rows: Record<string, unknown>[]): unknown | null;