Add LineCollection plot

[Illviljan]

This adds a line plotter based on LineCollections, called .lines.

I wanted to replace darray.plot() with using LineCollection instead. But unfortunately due to how many cases are supported (and tested in xarray) darray.plot() will continue using plt.plot.

xref:
#4820
#5622

[Illviljan]

Scatter vs. Lines:

ds = xr.tutorial.scatter_example_dataset(seed=42)
hue_ = "y"
x_ = "y"
size_="y"
z_ = "z"

fig = plt.figure()
ax = fig.add_subplot(1, 2, 1, projection='3d')
ds.A.sel(w="one").plot.lines(x=x_, z=z_, hue=hue_, linewidth=size_, ax=ax)
ax = fig.add_subplot(1, 2, 2, projection='3d')
ds.A.sel(w="one").plot.scatter(x=x_, z=z_, hue=hue_, markersize=size_, ax=ax)

[headtr1ck]

For my taste it uses a bit too much private matplotlib API, hopefully that doesn't make things complicated in the long run. Maybe you can ask over at matplotlib to make some of these things public API?

With so many special cases it is always difficult to figure out if everything was thought of. My checks did not reveal any critical problems, so I guess we leave it like this and wait for bug reports.

Thanks!

[Illviljan]

For my taste it uses a bit too much private matplotlib API, hopefully that doesn't make things complicated in the long run. Maybe you can ask over at matplotlib to make some of these things public API?

I agree, but it should work fine to reference plt.scatter if anything breaks. The code follows scatter very closely besides for 3d, having to make mypy happy and the black code style.

In my dream world matplotlib would add a similar function. plt.scatter is preferred over PathCollection and the same arguments can be used for adding a plt.lines over using LineCollection.

[Illviljan]

Here's an interesting plot we should be easily be able to do with Lines:

https://www.reddit.com/r/dataisbeautiful/comments/1d4ahr1/oc_north_atlantic_sea_surface_temperature_anomaly/

Would probably require fixing #8831 to get that nice looking datetime colorbar.

Haven't found the data yet, noting it for the future.

[headtr1ck]

Add an entry in whats-new and this is good to go! Thanks!

[dcherian]

This looks nice. I have two requests:

1. Can we add some info to the docstring about the difference between lines and line and how to choose between the two?
2. Can we please consider calling it line_collection instead? The one character difference between two very similar but slightly different functions is not friendly to users.

On second thought, there is indeed a lot of private API here. Are you sure this is a good idea? It feels like major technical debt. Perhaps this is best suited to an external MPL extension?

[Illviljan]

Can we add some info to the docstring about the difference between lines and line and how to choose between the two?

Done, I think?

Can we please consider calling it line_collection instead? The one character difference between two very similar but slightly different functions is not friendly to users.

Sure I don't mind a different name, but line_collection is also misleading, mpl's LineCollection and PathCollection is as basic as it gets and there's a lot of functionality xarray must have (datetime support for example) that they don't support. Therefore I was thinking to follow mpl's direction with a more distinct name, as scatter/_line do quite a bit more than just call the Collection:

PathCollection -> plt.scatter -> xr.plot.scatter
LineCollection -> _line -> xr.plot.?

On second thought, there is indeed a lot of private API here. Are you sure this is a good idea? It feels like major technical debt. Perhaps this is best suited to an external MPL extension?

_line is as close to a copy/paste of plt.scatter as it can be right now. Sure maybe I can rewrite it in a more public way, but then it'll be harder to use plt.scatter as a reference, which probably will be needed whenever someone finds a bug.

I think 3d plotting is the sort of thing that xarray should be good at, with scatter and lines a lot of the base needs should be covered.

[headtr1ck]

I'm fine with leaving it like this because currently there is really no nice way of rewriting it in a more "public" way.

@dcherian you can also discuss this in the bi-weekly if you feel like it.

[Illviljan]

https://www.reddit.com/r/dataisbeautiful/comments/1d4ahr1/oc_north_atlantic_sea_surface_temperature_anomaly/

Got this working with xarray:

# https://www.reddit.com/r/dataisbeautiful/comments/1d4ahr1/oc_north_atlantic_sea_surface_temperature_anomaly/#lightbox
# https://github.com/afinemax/climate_change_bot/blob/main/bot.py


import requests

import numpy as np
import matplotlib.pyplot as plt
import xarray as xr


def download_json(url):
    try:
        response = requests.get(url)
        response.raise_for_status()  # Raise an exception for unsuccessful responses (4xx, 5xx)
        json_data = response.json()
        return json_data
    except requests.exceptions.RequestException as e:
        print(f"Error downloading JSON: {e}")
        return None


url = r"https://climatereanalyzer.org/clim/sst_daily/json/oisst2.1_natlan1_sst_day.json"
sst_daily_data = download_json(url)


data = []
years = []
for i in range(len(sst_daily_data) - 3):
    years.append((sst_daily_data[i]["name"]))
    data.append(sst_daily_data[i]["data"])

# convert data from list into nd arr
data = np.asarray(data, dtype=float)  # first index is year, 2nd index is day

# ds = xr.Dataset(
#     data_vars=dict(temperature=(("year", "day"), data)),
#     coords=dict(
#         year=np.array(dict_names, dtype="datetime64[D]"),
#         day=np.arange(data.shape[-1], dtype="timedelta64[D]"),
#     ),
# )
ds = xr.Dataset(
    data_vars=dict(
        temperature=(
            ("year", "day"),
            data,
            dict(
                long_name="Daily North Atlantic (0-60N) Sea Surface Temperature (SST)",
                units="degC",
            ),
        )
    ),
    coords=dict(
        year=np.array(years, dtype=int),
        day=np.arange(data.shape[-1], dtype=int),
    ),
)

plt.figure()
ds.plot.lines(x="day", y="temperature", hue="year")

I couldn't get it working with datetimes though.

[headtr1ck]

Some want to merge this finally?
Or is there some discussion still required?