private double getChange(final TreeNode node) {
        final Double d = change.get(node);
        return d != null
                ? d.doubleValue()
                : 0;
    }
Returns the size of a level.
<p>
When the root is located at the top or bottom the size of a level is the
maximal height of the nodes of that level. When the root is located at
the left or right the size of a level is the maximal width of the nodes
of that level.

@param level
@return the size of the level [level >= 0 && level < levelCount]
stroom-core-client-widget/src/main/java/stroom/widget/htree/client/treelayout/AbegoTreeLayout.java