## Neal Grantham

[email protected]

# Draw DAGs with TikZ

I’m working my way through the Statistical Rethinking 2022 course by Richard McElreath and it contains a lot of Directed Acyclic Graphs (DAGs).

It got me wondering: how do I draw DAGs like those in the book?

The answer is TikZ, according to a tweet from McElreath where he calls it the “graphical language of defeat”. It really is not an easy language to pick up — the TikZ user manual is 1,321 pages long!

Thankfully, we don’t need to know much about TikZ to draw DAGs. I walk through the bare minimum you need to know in this post.

## What is TikZ?

TikZ (pronounced “ticks”) is a program for drawing graphical elements in LaTeX.

The name TikZ is a recursive acronym for “TikZ ist kein Zeichenprogramm,” which, translated from German, means “TikZ is no drawing program.”

Don’t let it fool you — TikZ very much is a drawing program. Just look at all these drawings made with TikZ!

For a gentle introduction to TikZ for drawing network diagrams (like DAGs) I recommend Crash Course to TikZ – Basics and Crash Course to Tikz – Positioning by Rachel Menghua Wu. The Wikibooks entry to LaTeX/PGF/TikZ is also helpful to understand the style options available in TikZ.

If you are making TikZ drawings for the web (as I have done in this post), you should configure your system to output TikZ drawings to Scalable Vector Graphics (SVGs). To do so, follow the directions in How to automatically convert TikZ images to SVG (with fonts!) from knitr by Andrew Heiss.1

## Baby’s First DAG

Let’s start with a very simple DAG: X affects Y .

Within `\tikz`, we specify three components. First, a node with `\node` we call `x` at the origin `(0,0)` which we label with `\$X\$`. Second, another node with `\node` we call `y` to the right of `x` at `(1,0)` which we label `\$Y\$`. Third, a path with `\path` between them as represented by an arrow `->` from `x` to `y`.

``````\tikz{
\node (x) at (0,0) {\$X\$};
\node (y) at (1,0) {\$Y\$};
\path[->] (x) edge (y);
}
``````

First impressions? Yeah, we can do better.

An SVG produced by TikZ has black foreground text and a transparent background by default.

Let’s lighten the foreground text. Define a new color which we call `offwhite`, use the `HTML` color model (so we can provide the color as a hexadecimal triplet), and finally, use the hexidecimal triplet `F2EDED` (the color I use for text throughout this website).

Now, add `offwhite` to each `\node` and `\path` to color the variables and arrow, respectively.

``````\definecolor{offwhite}{HTML}{F2EDED}
\tikz{
\node[offwhite] (x) at (0,0) {\$X\$};
\node[offwhite] (y) at (1,0) {\$Y\$};
\path[->, offwhite] (x) edge (y);
}
``````

Ok, a little better.

But that default arrowhead is… kinda ugly. We can change it by setting `>` to `stealth` within `\tikzset`.2

``````\definecolor{offwhite}{HTML}{F2EDED}
\tikzset{> = stealth}
\tikz{
\node[offwhite] (x) at (0,0) {\$X\$};
\node[offwhite] (y) at (1,0) {\$Y\$};
\path[->, offwhite] (x) edge (y);
}
``````

Not bad.

## Three’s Company

How about a DAG with three nodes?

Suppose X affects Y and Z is a confounder.

``````\definecolor{offwhite}{HTML}{F2EDED}
\tikzset{> = stealth}
\tikz{
\node[offwhite] (x) at (0,0) {\$X\$};
\node[offwhite] (y) at (2,0) {\$Y\$};
\node[offwhite] (z) at (1,1) {\$Z\$};
\path[->, offwhite] (x) edge (y);
\path[->, offwhite] (z) edge (x);
\path[->, offwhite] (z) edge (y);
}
``````

Nice!

Rather than set the style individually on each node and path, we can instead define styles within `\tikzset` that apply to `every node` and `every path` with `/.append style`.

For every node, we set `text` to `offwhite`. For every path, we set `arrows` to `->`, `draw` to `offwhite` (which colors the arrow “body”) and `fill` to `offwhite` (which colors the arrowhead).

``````\definecolor{offwhite}{HTML}{F2EDED}
\tikzset{
> = stealth,
every node/.append style = {
text = offwhite
},
every path/.append style = {
arrows = ->,
draw = offwhite,
fill = offwhite
}
}
\tikz{
\node (x) at (0,0) {\$X\$};
\node (y) at (2,0) {\$Y\$};
\node (z) at (1,1) {\$Z\$};
\path (x) edge (y);
\path (z) edge (x);
\path (z) edge (y);
}
``````

Looking good.

## Positioning

You may consider using the `positioning` library, which you can load with `\usetikzlibrary`.

Instead of specifying the exact position of nodes, you position each node in relation to another node with `right`, `left`, `above right`, `above left`, `below right`, and `below left`.

For example, in the previous DAG, we can define `x`, position `z` above and to the right of `x` with `above right = of x`, and position `y` below and to the right of `z` with `below right = of z`.

``````\usetikzlibrary{positioning}
\definecolor{offwhite}{HTML}{F2EDED}
\tikzset{
> = stealth,
every node/.append style = {
text = offwhite
},
every path/.append style = {
arrows = ->,
draw = offwhite,
fill = offwhite
}
}
\tikz{
\node (x) {\$X\$};
\node (z) [above right = of x] {\$Z\$};
\node (y) [below right = of z] {\$Y\$};
\path (x) edge (y);
\path (z) edge (x);
\path (z) edge (y);
}
``````

Structurally, this is exactly the same as the previous DAG. However, the text is smaller and the arrows are longer, almost as if we have “zoomed out” on the previous DAG.3

Here’s a more complicated DAG where the `positioning` library proves useful.

``````\usetikzlibrary{positioning}
\definecolor{offwhite}{HTML}{F2EDED}
\tikzset{
> = stealth,
every node/.append style = {
text = offwhite
},
every path/.append style = {
arrows = ->,
draw = offwhite,
fill = offwhite
}
}
\tikz{
\node (a) {\$A\$};
\node (z) [right = of a] {\$Z\$};
\node (b) [right = of z] {\$B\$};
\node (x) [below left = of z] {\$X\$};
\node (y) [below right = of z] {\$Y\$};
\node (c) [below right = of x] {\$C\$};
\path (a) edge (x);
\path (a) edge (z);
\path (b) edge (y);
\path (b) edge (z);
\path (c) edge (x);
\path (c) edge (y);
\path (x) edge (y);
\path (z) edge (x);
\path (z) edge (y);
}
``````

In practice you will find there are times to favor the `positioning` library and times where it’s better to stick to the manual approach of positioning nodes directly.

## Unobserved Variables

Throughout the Statistical Rethinking book, McElreath uses circles to represent variables that are unobserved.

To draw a circle around a node, we set `draw` to the color `offwhite` (the default is `none` which means no border), `shape` to `circle` (the default is `square`), and `inner sep` to `1pt` (so the circle radius is not too large).

We don’t want every node to appear this way, so define a style called `hidden` (you can name it whatever you want) with these settings that we apply only to unobserved variables in the DAG.

``````\usetikzlibrary{positioning}
\definecolor{offwhite}{HTML}{F2EDED}
\tikzset{
> = stealth,
every node/.append style = {
draw = none,
text = offwhite
},
every path/.append style = {
arrows = ->,
draw = offwhite,
fill = offwhite
},
hidden/.style = {
draw = offwhite,
shape = circle,
inner sep = 1pt
}
}
\tikz{
\node (x) {\$X\$};
\node[hidden] (z) [above right = of x] {\$Z\$};
\node (y) [below right = of z] {\$Y\$};
\path (x) edge (y);
\path (z) edge (x);
\path (z) edge (y);
}
``````

And here’s a more complicated DAG (from Homework 3) with an unobserved variable. Because the positions of nodes don’t easily map to a grid, we don’t use the `positioning` library and instead specify the exact positions of each node.

``````\definecolor{offwhite}{HTML}{F2EDED}
\tikzset{
> = stealth,
every node/.append style = {
text = offwhite
},
every path/.append style = {
arrows = ->,
draw = offwhite,
fill = offwhite
},
hidden/.style = {
draw = offwhite,
shape = circle,
inner sep = 1pt
}
}
\tikz{
\node (a) at (0,0) {\$A\$};
\node (s) at (0,2) {\$S\$};
\node (x) at (1,1) {\$X\$};
\node (y) at (2.5,1) {\$Y\$};
\node[hidden] (u) at (1.5,2) {\$U\$};
\path (a) edge (s);
\path (a) edge (x);
\path (a) edge (y);
\path (s) edge (x);
\path (s) edge (y);
\path (x) edge (y);
\path (u) edge (s);
\path (u) edge (y);
}
``````

## That’s all, folks

You should have everything you need to draw DAGs like those in Statistical Rethinking. If there are other DAG features you’d like to see, let me know on Twitter.

Until then, have a wonderful DAG.

1. I ran into difficulties trying to connect `dvisvgm` to Ghostscript on macOS. I found that reinstalling MacTex with “Ghostscript Dynamic Library” enabled didn’t work for me — `dvisvgm` wouldn’t recognize the dynamic library at `/usr/local/share/ghostscript/9.53.3/lib/libgs.dylib.9.53` despite properly pointing to it with the `LIBGS` environment variable. However, you can `brew install ghostscript` and then point `dvisvgm` to the dynamic library at `/usr/local/Cellar/ghostscript/9.55.0/lib/libgs.dylib.9.55` and that works. No clue why. In any case, I never would have figured out all these steps on my own. Thanks Andrew!

2. Why is it called `stealth`? My best guess is that the resulting arrowhead looks vaguely like a Stealth Bomber.

3. I haven’t found a way to “zoom back in”. If you know how, do tell.

February 9, 2022  @nsgrantham