@@ -10,6 +10,9 @@ build:
     post_create_environment:
       - pip install PyQt5 setuptools numpy Cython
       - python setup.py build_ext --inplace
+    pre_install:
+      # Development builds may require dependency versions not yet released on PyPI
+      - python scripts/rtd_install_dev_deps.py
 sphinx:
    configuration: doc/conf.py
 formats:
 
@@ -0,0 +1,135 @@
+# -*- coding: utf-8 -*-
+"""
+Install development versions of dependencies for Read the Docs builds.
+
+This project is developed alongside other PlotPyStack and DataLab Platform
+projects: development branches and pull requests may require dependency versions
+which are not yet published on PyPI, so installing the documentation requirements
+from PyPI alone would fail.
+
+This script mirrors the GitHub Actions test workflows:
+
+- tag builds and release branches: nothing is done, dependencies are installed
+  from PyPI by the regular ``python.install`` step;
+- other branches and pull requests: the dependencies listed in
+  ``DEV_DEPENDENCIES`` are installed from GitHub. The regular ``python.install``
+  step then keeps these versions as they already satisfy the project
+  requirements.
+
+It is intended to be run as a ``build.jobs.pre_install`` step and only relies on
+the standard library and ``git``.
+
+The same script is used by several projects: only the project-specific settings
+below differ between copies.
+"""
+
+from __future__ import annotations
+
+import os
+import subprocess
+import sys
+
+# ---- Project-specific settings ----------------------------------------------
+PROJECT_REPO = "https://github.com/PlotPyStack/PlotPy.git"
+#: Placeholder for the name of the branch being built
+SAME_BRANCH = "{branch}"
+#: Dependencies to install from GitHub for development builds:
+#: package name -> (repository URL, candidate branches, first existing one is used)
+DEV_DEPENDENCIES = {
+    "PythonQwt": ("https://github.com/PlotPyStack/PythonQwt.git", ("master",)),
+    "guidata": ("https://github.com/PlotPyStack/guidata.git", ("develop",)),
+}
+# -----------------------------------------------------------------------------
+
+RELEASE_BRANCHES = ("main", "master", "release")
+
+
+def get_remote_branches(url: str) -> dict[str, str]:
+    """Return the branches of a remote Git repository.
+
+    Args:
+        url: Remote repository URL
+
+    Returns:
+        Mapping of branch names to the commit hash of their head
+    """
+    output = subprocess.run(
+        ["git", "ls-remote", "--heads", url],
+        capture_output=True,
+        text=True,
+        check=True,
+    ).stdout
+    branches = {}
+    for line in output.splitlines():
+        commit, ref = line.split("\t", 1)
+        branches[ref.removeprefix("refs/heads/")] = commit
+    return branches
+
+
+def get_built_branch() -> str | None:
+    """Return the branch being built, if it can be determined.
+
+    For pull request builds, Read the Docs only provides the pull request number,
+    so the branch is the one of the project repository whose head is the commit
+    being built.
+
+    Returns:
+        Branch name, or None for tag builds and pull requests from forks
+    """
+    version_type = os.environ.get("READTHEDOCS_VERSION_TYPE")
+    if version_type == "branch":
+        return os.environ["READTHEDOCS_GIT_IDENTIFIER"]
+    if version_type == "external":
+        commit = os.environ["READTHEDOCS_GIT_COMMIT_HASH"]
+        for branch, head in get_remote_branches(PROJECT_REPO).items():
+            if head == commit:
+                return branch
+    return None
+
+
+def select_branch(url: str, candidates: tuple[str, ...], built: str | None) -> str:
+    """Return the first candidate branch which exists in a remote repository.
+
+    Args:
+        url: Remote repository URL
+        candidates: Candidate branch names, ``SAME_BRANCH`` standing for the
+         branch being built
+        built: Branch being built, or None if unknown
+
+    Returns:
+        Selected branch name
+    """
+    names = [built if name == SAME_BRANCH else name for name in candidates]
+    existing = get_remote_branches(url)
+    for name in names:
+        if name is not None and name in existing:
+            return name
+    raise SystemExit(f"None of the branches {names} exists in {url}")
+
+
+def main() -> None:
+    """Install dependencies from GitHub for development builds."""
+    version_type = os.environ.get("READTHEDOCS_VERSION_TYPE")
+    if version_type not in ("branch", "external"):
+        print(f"Version type {version_type!r}: dependencies installed from PyPI")
+        return
+    built = get_built_branch()
+    if built in RELEASE_BRANCHES:
+        print(f"Branch {built!r}: dependencies installed from PyPI")
+        return
+    print(f"Development build (branch: {built or 'unknown'})")
+    requirements = []
+    for name, (url, candidates) in DEV_DEPENDENCIES.items():
+        branch = select_branch(url, candidates, built)
+        print(f"  {name}: {branch!r} branch")
+        requirements.append(f"{name} @ git+{url}@{branch}")
+    # Single pip call, so that dependencies between these packages are resolved
+    # against their development versions rather than against PyPI releases
+    subprocess.run(
+        [sys.executable, "-m", "pip", "install", "--no-cache-dir", *requirements],
+        check=True,
+    )
+
+
+if __name__ == "__main__":
+    main()