findTopCellItem() handle underflow case

Meter.c

@@ -408,11 +408,17 @@ static uint8_t GraphMeterMode_findTopCellItem(const Meter* this, double scaledTo
 
       double topPoint = (valueSum / scaledTotal) * (double)(int)graphHeight;
       double area = (value / scaledTotal) * (double)(int)graphHeight;
-      assert(area >= 0.0); // "area" can be 0.0 when the division underflows
 
-      if (topPoint - (double)(int)topCell > 0.0) {
-         if (area > topPoint - (double)(int)topCell)
+      if (topPoint - (double)(int)topCell > 0.0 || (topPoint <= 0.0 && topCell < 1)) {
+         // If the division underflows (topPoint <= 0.0 && topCell < 1), that
+         // item still counts as part of the top cell ("area" would be 0.0 in
+         // this case).
+
+         // Cut the excess area
+         if (area > topPoint - (double)(int)topCell) {
             area = topPoint - (double)(int)topCell;
+         }
+         assert(area >= 0.0);
 
          // Find the item that occupies the largest area of the top cell.
          // Favor item with higher index in case of a tie.