amt = n_cells * avg_pct * avg_suit
Amount (amt) is the multiplication of the number of cells (n_cells),
average percent (avg_pct) of a cell's contents within the selected polygon,
and the average Suitability (avg_suit; 0 to 100%) of the species given by AquaMaps.
Note: The rendering of this treemap plot is SLOW if # species > 1,000. Use toolbar on left of map to draw a smaller area with fewer species.