svgling: Linguistic tree diagrams in python + svg

Author: Kyle Rawlins, kgr@jhu.edu

Website and documentation: https://rawlins.github.io/svgling/

Dependencies: svgwrite, python 3, (optional) cairosvg

Repository: https://github.com/rawlins/svgling/

Installation: pip install svgling for the current release version, install from the repository for the current unreleased version. (E.g., pip install git+https://github.com/rawlins/svgling)

License: MIT License

Overview

The svgling package is a pure python package for doing single-pass rendering of tree diagrams as used in Linguistics and NLP, in the SVG (Scalable Vector Graphics) format. It is primarily intended for integrating with Jupyter notebooks, but can also be used to generate SVG diagrams for all sorts of other purposes. It involves no javascript and so will work in Jupyter without any plugins.

The basic interface is pretty simple: pass a tree-describing object to svgling.draw_tree (e.g. a tuple consisting of a label and a sequence of daughter nodes, which may themselves be trees).

import svgling
svgling.draw_tree(("S", ("NP", ("D", "the"), ("N", "elephant")), ("VP", ("V", "saw"), ("NP", ("D", "the"), ("N", "rhinoceros")))))

In Jupyter, this code produces an SVG image like the following:

The tree drawing code accepts two main tree formats: lisp-style trees made from lists of lists (or tuples of tuples), with node labels as strings, or trees from the nltk package, i.e. objects instantiating the nltk.tree.Tree API. The following nltk code run in Jupyter, as long as svgling has been installed, produces an identical tree diagram to the above example:

nltk.Tree.fromstring("(S (NP (D the) (N elephant)) (VP (V saw) (NP (D the) (N rhinoceros))))")

On current versions of nltk, the support goes both ways: the default (and only) tree renderer in Jupyter is svgling. If the package is installed, nltk Trees will automatically render using svgling trees. For more control, you can also provide a tree directly to the svgling.draw_tree function, which allows you to use options:

x = nltk.Tree.fromstring("(S (NP (D the) (N elephant)) (VP (V saw) (NP (D the) (N rhinoceros))))")
svgling.draw_tree(x, leaf_nodes_align=True)

Beyond basic tree-drawing, the package supports a number of flourishes like movement arrows. For documentation and examples, see the project website:

• Overview of the package
• svgling diagram gallery
• Full manual for svgling

These three pages are directly rendered from notebooks in the [docs/] (https://github.com/rawlins/svgling/tree/master/docs) directory that you can also download and execute in Jupyter Lab (or another frontend, modulo some compatibility notes).

Core design principles and goals

1. Be well suited for programmatic generation of tree diagrams (not just hand-customized diagrams).
2. Be equally suited for theoretical linguistics and computational linguistics/NLP, at least for cases where the latter is targeting constituent trees. (This package is not aimed at dependency trees/graphs.)
3. Do as much as possible with pure python (as opposed to python+javascript, or python+tk, or python+dot, or...).

Strengths and limitations

The svgling package does its rendering in one pass -- it takes a tree structure as input, produces an svg output, and that's it. Because of this, it is extremely simple to use in Jupyter, and no messing with plugins or Jupyter settings should be necessary. Because it is SVG-based, scaling and embedding in any web context should work smoothly. It also has minimal dependencies, just one package (svgwrite) that provides an abstraction layer over the process of generating valid SVG text.

Single-pass rendering also places limitations on what can be done. One of the challenges is that it mostly uses absolute position, and the exact position and width of text elements can't be determined without actually rendering to some device and seeing what happens. In addition, the exact details of rendering are in various ways at the mercy of the rendering device. This all means that svgling uses a bunch of tricks to estimate node size and width, and won't always be perfect on all devices. This situation also places some hard limitations on how far svgling can be extended without adding javascript or other multi-pass rendering techniques. For example, I would eventually like to allow mathjax in nodes, but at the moment this does not seem possible in pure SVG without javascript on the client side. The package does provide support for hybrid HTML/SVG tree diagrams that allows MathJax in nodes, but this rendering mode comes with certain limitations.

There are many things that it might be nice to add to this package; if you find svgling useful, have any requests, or find any bugs, please let me know.

Compatibility

The SVG files produced by svgling should be compatible with all major browsers (Chrome, Firefox, Safari, Edge) on both desktop and mobile; if you find a compatibility issue with some browser, please report it as a bug. It also supports all major interactive editing packages that I am aware of.

There are various ways to convert the generated SVG files to other formats, including raster ones, but the recommended/supported way to do this programmatically is via the cairosvg package. svgling.utils provides some convenience functions for conversion to PNG/PDF/PS using cairosvg. See the "Compatibility and Conversion" section of the svgling manual for more details. Note that cairosvg is an optional dependency of svgling, so won't necessarily be installed by default.