def get_cell(row_index, column_index)
      if within_range?(row_index, column_index)
        @rows[row_index][column_index].v
      else
        raise RangeError, "row_index and column_index MUST be < @rows.size and @cols.size", caller
      end
    end
Gets a cell value from the visualization, at row_index, column_index. row_index and column_index start from 0.

Parameters:
* row_index       [Required] row_index starts from 0.
* column_index    [Required] column_index starts from 0.