Background

The WCSAxes package was developed with the intent that it would some day be merged into the core Astropy package. Since it is now stable, a number of people have asked for it to be merged. It would be nice to include this in the 1.3 release!

Update: this is now ready for review! You can view the rendered documentation here:

http://astropy-test.readthedocs.io/en/latest/visualization/wcsaxes/index.html

Once #5506 is merged, we can update the WCSAxes tests so that they get run in --remote-data=astropy mode.

Open questions

• Any objections to include WCSAxes in Astropy?

• Any objections to including the full git history? (note that any PNG files have been purged from the history, and issue numbers have been converted to astrofrog/wcsaxes#... to avoid auto-closing issues).

• Is astropy.visualization.wcsaxes the right place to put this? Alternatively, it could be a top-level package astropy.wcsaxes for more exposure. If we keep it at astropy.visualization.wcsaxes, do we still want to consider having it in the top-level package list anyway?

• How should we deal with the optional Matplotlib dependency? It's hard to hide away all the imports inside classes, so should we simply raise an error if astropy.visualization.wcsaxes is imported?

• Are people happy to say that we support the last two stable releases of Matplotlib? (this is important for CI testing)

Things that need doing before this is merged

• Read over and update documentation
• Fix documentation warnings
• Fix tests
• Get rid of absolute imports to Astropy in the WCSAxes code
• Fix plot_directive config for plots in documentation - we had custom settings in WCSAxes and we need to copy that over
• Add instructions on adding image tests
• Deal with astropy.visualization.wcsaxes.datasets - this should probably be replaced by functions from astropy.utils.data
• Add proper 'getting started' example in documentation
• Check for commit messages that might auto-close issues and update accordingly
• Purge the WCSAxes history of reference PNG files (we used to store the PNG files in the repo)

Things that can be done after this is merged

• Update WCSAxes docs to redirect to Astropy docs
• There are a couple of places in Astropy where there are comments about things becoming possible after WCSAxes is merged in, so need to check those.
• Write up instructions for merging a package like this into Astropy
• Refactor code to make a better use of Angle class (see comments by @mhvk)
• Open issues for TODO/FIXME entries

Details

This merge preserves the full version history of WCAxes. To do this, I made a special branch in the WCSAxes repo which has all the files moved to the correct position and all the imports updated for instance: https://github.com/astrofrog/wcsaxes/tree/astropy-merge

I am then adding a few more commits in this PR that relate to updating Astropy following the merge. The merge was done with

git remote add wcsaxes git@github.com:astrofrog/wcsaxes.git
git fetch wcsaxes
git merge wcsaxes/astropy-merge --allow-unrelated-histories

I also did some things in the astropy-merge branch to get rid of PNG files in the history and also changed any issue numbers in commit messages to explicitly point to astrofrog/wcsaxes#... to avoid auto-closing issues. I will write up the whole process after this is merged (if it is).