layout method
给定一个区域范围和节点,对该节点的Children进行布局(不包含孩子的孩子)
Implementation
@override
void layout(TreeMapNode root, Rect area) {
root.children.sort((a, b) {
return b.value.compareTo(a.value);
});
layoutChildren(ratio, root, area.left, area.top, area.right, area.bottom);
}