File: CONTRIBUTING.md

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## Feedback and Contribution

We welcome any input, feedback, bug reports, and contributions via [Altair's
GitHub Repository](http://github.com/altair-viz/altair/). 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](https://bit.ly/join-vega-slack).


### 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](https://altair-viz.github.io/gallery/index.html) 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](https://vega.github.io/vega-lite/examples/) which are missing
in the altair gallery. Or you can feel free to be creative and build your own
visualizations.


#### Initial Setup
Install the latest version of Altair locally using 
```
pip install git+https://github.com/altair-viz/altair/
```
Next step is to fork the repository on GitHub and clone the fork to you local
machine. For more details on forking see the [GitHub
Documentation](https://help.github.com/en/articles/fork-a-repo).
```
git clone https://github.com/YOUR-USERNAME/altair.git
```
You can have a single clone of the repository that points to both your fork and
the main package repository. These pointers to GitHub are called "remotes" .
On your local clone you should run:
```
git remote add upstream https://github.com/altair-viz/altair.git
git checkout master
git pull upstream master
```
And then you'll have all the updates in the master branch of your local fork.
Note that git will complain if you've committed changes to your local master
branch that are not on upstream (this is one reason it's good practice to **never**
work directly on your master branch). 

#### Steps to Contribute

Often it is convenient to draft an example outside of the main repository, such
as [Google Colab](https://colab.research.google.com/), to avoid difficulties
when working with git. Once you have an example you would like to add the procedure is
to first update your master branch. 
```
git checkout master
git pull upstream master
```
Then create a branch for your example submission
```
git checkout -b <branch-name>
```
Now you are on a new branch where you can add your example file. All examples
files are kept in the `altair/examples/` directory. Make sure the style and
format match the other examples.  An official style guide for examples has not
been created yet, but a few suggestions are 
- The file docstring will be rendered into HTML via
  [reStructuredText](http://docutils.sourceforge.net/rst.html), 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. 
- 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.

Next step is to commit your example file and push the branch to your forked repo
```
git add <example.py>
git commit -m 'DOC: Add example for <example>'
git push origin <branch-name>
```

Finally you will need to submit a pull request (PR) on GitHub asking to merge
your example branch into altair master. For details on creating a PR see GitHub
documentation [Creating a pull
request](https://help.github.com/en/articles/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 example will
help others in the future. Note that examples shown on the [altair
website](https://altair-viz.github.io/) are only updated when a new version is
released so your new example might not show up there for a while.