Data-grid heatmap (type: 'heatmap') — not scatter density.
Static peaks mode and a live spectrogram that streams columns via
chart.updateHeatmap({ mode: 'appendColumns', scrollX: true })
(GPU modular ring + O(rows) strip upload).
updateHeatmap column-append (not per-frame full setOption).
appendData remains unsupported for heatmap — wrong payload model (points vs grid).
Style controls keep the same data object identity so the stream is not cleared.