Array of per-cell shape overlays drawn on top of standard heatmap cells. Each entry is an object with a *variable* and *sample* name identifying the cell, plus optional *shape*, *color*, and *size* (fraction of the cell, 0..1). Example: [{variable: "V1", sample: "S3", shape: "pch4"}] to cross out a cell with an X. Common shapes: pch4 (X), pch3 (plus), pch1 (open circle), pch0 (open square), pch5 (open diamond), pch16 (filled circle), circle, square, diamond
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