SimMath’s Grid is meant to divide space up into cells. It’s not meant to be used as the model for some dynamically sized UI. It uses integers to partition space because it makes it easier to create zone IDs… and partitioning space on non-integer boundaries seemed like a rare case.
If you are just looking for a row/column data structure then maybe use the Table class from Guava. (Lemur already depends on Guava so you already have it.)