Consistent tree drawing

In his VizBi 2011 presentation, Rod Page discussed how the same tree can be drawn in different ways. This is a problem as tree may display differently between viewers, and following the addition or removal of leaves.

Image extracted from Rod's VizBi 2011 presentation

He showed how leaves can be ordered using sequential ids, for example from the NCBI taxonomy. This ordering is then used to specify how leaves are be displayed in the Y dimension leading to consistent display between viewers. In addition, this ordering is not affected by the addition of new, or deletion of existing, leaves.

Image extracted from Rod's VizBi 2011 presentation

Ordering from an arbitrarily ordered list, such as the sequence of names added to the NCBI taxonomy, results in an arbitrarily node ordering. Yet leaves are often names, not numbers, and we are used to viewing text in alphabetical order (a-z).

Leaves may, however, be ordered alphabetically when a tree is first built. To prevent restructuring of a tree, following the addition of new groups of leaves, new groups can be appended to the initial list in alphabetical order. Such ordering maintains consistency in tree rendering while supporting partial alphabetical ordering of tips.

Image edited from Rod's VizBi 2011 presentation

When desired the sequential alphabetically ordered groups can be sorted to create a new fully ordered list.

Image edited from Rod's VizBi 2011 presentation

Whether such partial alphabetical ordering improves tree comprehension requires evaluation, but it is likely to be most beneficial for trees with many polytomies, e.g. taxonomies. There are also overheads associated with maintaining the ordered list(s) with trees to be considered.