CONTRIBUTING.md

Feedback and Contribution

We welcome any input, feedback, bug reports, and contributions via Altair's GitHub Repository. In particular, we welcome companion efforts from other visualization libraries to render the Vega-Lite specifications output by Altair. We see this portion of the effort as much bigger than Altair itself: the Vega and Vega-Lite specifications are perhaps the best existing candidates for a principled lingua franca of data visualization.

We are also seeking contributions of additional Jupyter notebook-based examples in our separate GitHub repository: https://github.com/altair-viz/altair_notebooks.

The altair users mailing list can be found at https://groups.google.com/forum/#!forum/altair-viz. If you are working on Altair, you can talk to other developers in the #altair channel of the Vega slack.

How To Contribute Code to Altair

Setting Up Your Environment

Fork the Altair repository on GitHub and clone the fork to you local machine. For more details on forking see the GitHub Documentation.

$ git clone https://github.com/YOUR-USERNAME/altair.git

To keep your fork up to date with changes in the this repo, you can use the fetch upstream button on GitHub.

Now you can install the latest version of Altair locally using pip. The -e flag indicates that your local changes will be reflected every time you open a new Python interpreter (instead of having to reinstall the package each time).

$ cd altair/ 
$ python -m pip install -e .

You can now install the development requirements using

$ python -m pip install -r requirement_dev.txt

Creating a Branch

Once your local environment is up-to-date, you can create a new git branch which will contain your contribution (always create a new branch instead of making changes to the master branch):

$ git switch -c <branch-name>

With this branch checked-out, make the desired changes to the package.

Note that Altair code uses the black code formatter and flake8 linter which you can apply to your modifications by running:

$ black --diff .  # View changes
$ black .  # Apply changes
$ flake8 .  --statistics  # View changes (fix manually)

Testing your Changes

Before suggesting your contributing your changing to the main Altair repository, it is recommended that you run the Altair test suite, which includes a number of tests to validate the correctness of your code:

$ make test

Study the output of any failed tests and try to fix the issues before proceeding to the next section.

Creating a Pull Request

When you are happy with your changes, you can commit them to your branch by running

$ git add <modified-file>
$ git commit -m "Some descriptive message about your change"
$ git push origin <branch-name>

You will then need to submit a pull request (PR) on GitHub asking to merge your example branch into the main Altair repository. For details on creating a PR see GitHub documentation Creating a pull request. You can add more details about your example in the PR such as motivation for the example or why you thought it would be a good addition. You will get feed back in the PR discussion if anything needs to be changed. To make changes continue to push commits made in your local example branch to origin and they will be automatically shown in the PR.

Hopefully your PR will be answered in a timely manner and your contribution will help others in the future.

Adding Examples

We are always interested in new examples contributed from the community. These could be everything from simple one-panel scatter and line plots, to more complicated layered or stacked plots, to more advanced interactive features. Before submitting a new example check the Altair Example Gallery to make sure that your idea has not already been implemented.

Once you have an example you would like to add there are a few guide lines to follow. Every example should:

• be saved as a stand alone script in the altair/examples/ directory.
• have a descriptive docstring, which will eventually be extracted for the documentation website.
• contain a category tag.
• define a chart variable with the main chart object (This will be used both in the unit tests to confirm that the example executes properly, and also eventually used to display the visualization on the documentation website).
• not make any external calls to download data within the script (i.e. don't use urllib). You can define your data directly within the example file, generate your data using pandas and numpy, or you can use data available in the vega_datasets package.

The easiest way to get started would be to adapt examples from the Vega-Lite example gallery which are missing in the Altair gallery. Or you can feel free to be creative and build your own visualizations.

Often it is convenient to draft an example outside of the main repository, such as Google Colab, to avoid difficulties when working with git. Once you have an example you would like to add, follow the same contribution procedure outlined above.

Some additional notes:

• all examples should be in their own file in the altair/examples directory, and the format and style of new contributions should generally match that of existing examples.
• The file docstring will be rendered into HTML via reStructuredText, so use that format for any hyperlinks or text styling. In particular, be sure you include a title in the docstring underlined with ---, and be sure that the size of the underline exactly matches the size of the title text.
• If your example fits into a chart type but involves significant configuration it should be in the Case Studies category. If your example doesn't fit well into any category then it can be included in the Other Charts category.
• For consistency all data used for a visualization should be assigned to the variable source. Then source is passed to the alt.Chart object. See other examples for guidance.
• Example code should not require downloading external datasets. We suggest using the vega_datasets package if possible. If you are using the vega_datasets package there are multiple ways to refer to a data source. If the dataset you would like to use is included in local installation (vega_datasets.local_data.list_datasets()) then the data can be referenced directly, such as source = data.iris(). If the data is not included then it should be referenced by URL, such as source = data.movies.url. This is to ensure that Altair's automated test suite does not depend on availability of external HTTP resources.

Note that examples shown on the Altair website are only updated when a new version is released so your new example might not show up there for a while.