git log: show second parent on the left

Question

Consider this example git repository.

#!/bin/bash -e
rm -rf example
git init example
cd example
echo 0 > file.txt
git add file.txt
git commit -am "initial commit"
git branch branch1
echo 1 > file.txt
git commit -am "1 (on master)"
git branch branch2
git checkout branch1
echo 2 > file2.txt
git add file2.txt
git commit -m "2 (on branch1)"
git merge --no-edit master
echo 3 > file2.txt
git commit -am "3 (on branch1)"
git checkout master
echo 4 > file.txt
git commit -am "4 (on master)"
git checkout branch1
git merge --no-edit master
echo 5 > file2.txt
git commit -am "5 (on branch1)"
git checkout master
git merge --no-edit --no-ff branch1
git log --graph --oneline

We create a branch branch1, merge up to it from the master branch a couple of times, and finally merge back to master.

git log --oneline --graph looks weirdly twisted.

*   2c3b97a Merge branch 'branch1'
|\
| * 1d722df 5 (on branch1)
| *   54039b6 Merge branch 'master' into branch1
| |\
| |/
|/|
* | d80d82c 4 (on master)
| * 2399286 3 (on branch1)
| *   9a0cb38 Merge branch 'master' into branch1
| |\
| |/
|/|
* | 31df841 1 (on master)
| * 5809072 2 (on branch1)
|/
* 08fc7a6 initial commit

The graph would be simpler if it looked like this:

*   2c3b97a Merge branch 'branch1'
|\
| * 1d722df 5 (on branch1)
| *   54039b6 Merge branch 'master' into branch1
|/|
* | d80d82c 4 (on master)
| * 2399286 3 (on branch1)
| *   9a0cb38 Merge branch 'master' into branch1
|/|
* | 31df841 1 (on master)
| * 5809072 2 (on branch1)
|/
* 08fc7a6 initial commit

The reason it doesn't look like that, as I understand it, is that git log always prefers to show the "second parent" of each merge commit on the right-hand side, so for each merge commit it has to snake around to the right. (The "second parent" is the branch merging in; the "first parent" is the branch we're merging into.)

1. Is there a way to convince git log to generate the simpler graph I'd prefer? There's an old gist from 2013 proposing fixing this ("Let me specify which branches sink to the left"; there are other interesting ideas there, too), but I see no progress on that.
2. Assuming git log can't do it naturally, is there another git commit graphing tool/library I can use to make a graph like this?

Answer 1

Thanks to torek's suggestion to try dot (aka Graphviz) I wrote this Python script to generate output in the dot file format. It generates a graph like this, which looks pretty good to me:

#!/usr/bin/env python3
import subprocess, sys, re

args = ['git', 'log', '--pretty=format:%x00%H%x01%h%x01%P%x00'] + sys.argv[1:]

rows = subprocess.check_output(args).split(b'\x00')[1::2]

seen_commits = set([row.split(b'\x01')[0].decode() for row in rows])

print("digraph G {")
for row in rows:
    columns = row.split(b'\x01')
    commit = columns[0].decode()
    label = columns[1].decode()
    parents = columns[2].decode().split(' ')
    print('"{}" [label="{}"];'.format(commit, label))
    for parent in parents:
        if parent == "":
            continue
        if not parent in seen_commits:
            continue
        print('"{}" -> "{}"'.format(commit, parent))
print("}")

If you put that file on your $PATH as git-log-dot and chmod +x git-log-dot, git will automatically run it if you type git log-dot. (It's a legacy holdover from when git was distributed as separate scripts, git-checkout, git-reset, etc.)

You'll also need Graphviz. On macOS I installed it with brew install graphviz; on Ubuntu I believe you can install it with sudo apt-get graphviz.

You can generate a PNG file using git log-dot like so:

git log-dot | dot -Tpng -o graph.png

(In the sample above, I also used -Grankdir=LR to make the graph horizontal, but the default vertical orientation is pretty good, too.)