Add an embedded basic chart (column, line, or bar) to a Google Sheet, overlaid on a sheet or placed on a new sheet.

Documented steps

1. POST https://sheets.googleapis.com/v4/spreadsheets/{spreadsheetId}:batchUpdate using OAuth scope https://www.googleapis.com/auth/spreadsheets.
2. Add requests[0].addChart.chart.spec.title (string) and requests[0].addChart.chart.spec.basicChart.chartType (e.g. "COLUMN", "LINE", "BAR") — ChartSpec also supports pieChart/histogramChart/etc., but only one chart-type field should be set.
3. Set basicChart.domains = [{domain:{sourceRange:{sources:[GridRange for the category/X-axis column]}}}] and basicChart.series = [{series:{sourceRange:{sources:[GridRange for a data column]}}, targetAxis:"LEFT_AXIS"}] (one series entry per data column, each with its own GridRange).
4. Set basicChart.headerCount = 1 if the domain/series source ranges include a header row that should be excluded from the plotted data and used for labels.
5. Set chart.position to {"newSheet": true} to place the chart on its own new sheet, or to {"overlayPosition": {"anchorCell": {sheetId, rowIndex, columnIndex}, "offsetXPixels": n, "offsetYPixels": n}} to embed it over an existing sheet at a specific cell anchor.
6. Docs: https://developers.google.com/workspace/sheets/api/reference/rest/v4/spreadsheets/request | https://developers.google.com/workspace/sheets/api/samples/charts | https://developers.google.com/workspace/sheets/api/limits