Add LineCollection plot #7173

Illviljan · 2022-10-16T20:16:28Z

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.

Examples:

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

# Datetimes not working for some unrelated reason:
# 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")

Calling it lines since scatter is not called path_collection:

PathCollection (Very low-level, e.g. no datetime handling) -> plt.scatter (Handles datetime etc.)-> xr.plot.scatter
LineCollection (Very low-level, e.g. no datetime handling) -> _line (Handles datetime etc. Pretty much a copy of plt.scatter) -> xr.plot.lines

Seaborns new object-based plotting uses lines as well with similar argument:
https://seaborn.pydata.org/generated/seaborn.objects.Line.html
https://seaborn.pydata.org/generated/seaborn.objects.Lines.html

xref:
#4820
#5622

Illviljan · 2022-10-17T13:57:47Z

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)

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Illviljan · 2024-12-20T21:08:06Z

Felt the need for this one again. I'm going to self-merge next week unless someone has any further thoughts.

headtr1ck · 2025-01-01T12:46:56Z

Felt the need for this one again. I'm going to self-merge next week unless someone has any further thoughts.

Let's go

zachjweiner · 2025-01-01T18:16:19Z

I tried out this PR, as it implements something similar to (but not quite) what I was looking for. Is this interface meant only to support variable sizes and colors based on a coordinate/variable, like scatter? I noticed that float values for linewidth are not handled/excepted on properly (and linewidths and lw are both ignored). I personally would never use variable-based linewidths - usually for such plots with many lines I just want to set them to be thin. (I would also likely make use of manually setting colors by passing a list thereof.)

My (very frequent) use case here is just that of DataArray.line.plot(hue=...), but adding a colorbar rather than a legend (which I currently do manually). Is it feasible to refactor any of the implementation here to enable something like this? (I also frequently create discrete colorbars, so that they're really just legends but in colorbar form. The levels argument gets close to this, except they aren't centered on the actual coordinate values.) Can move this to a new issue if this would be better as a separate feature request.

dcherian · 2025-01-02T23:34:11Z

my only concern is the one character difference between .line and .lines. Shall we call it line_collection instead? Ideally we would have only one of these but I can see its hard to do that now, given how complex the plotting module is.

Illviljan · 2025-01-07T21:15:56Z

I tried out this PR, as it implements something similar to (but not quite) what I was looking for. Is this interface meant only to support variable sizes and colors based on a coordinate/variable, like scatter? I noticed that float values for linewidth are not handled/excepted on properly (and linewidths and lw are both ignored). I personally would never use variable-based linewidths - usually for such plots with many lines I just want to set them to be thin. (I would also likely make use of manually setting colors by passing a list thereof.)

It's intended to be similar to scatter, yes. I've found the same arguments for using size on scatterplots can be made for lineplots.
I'll look into linewidth, linewidths and lw, during this week. Does scatter have similar issues?
But don't expect this to work exactly like plt.plot and xr.plot.line, they simply have too many options to replicate with LineCollection (I've tried, it got very messy).

My (very frequent) use case here is just that of DataArray.line.plot(hue=...), but adding a colorbar rather than a legend (which I currently do manually). Is it feasible to refactor any of the implementation here to enable something like this? (I also frequently create discrete colorbars, so that they're really just legends but in colorbar form. The levels argument gets close to this, except they aren't centered on the actual coordinate values.) Can move this to a new issue if this would be better as a separate feature request.

My idea for quick discrete colorbars is to simply convert the hue variable to string.

Illviljan · 2025-01-07T21:28:33Z

my only concern is the one character difference between .line and .lines. Shall we call it line_collection instead? Ideally we would have only one of these but I can see its hard to do that now, given how complex the plotting module is.

line_collection implies a more basic version in my mind:

PathCollection (Very low-level, e.g. no datetime handling) -> plt.scatter (Handles datetime etc.)-> xr.plot.scatter
LineCollection (Very low-level, e.g. no datetime handling) -> _line (Handles datetime etc. Pretty much a copy of plt.scatter) -> xr.plot.lines

Seaborns new object-based plotting uses lines as well with similar argument, so I don't feel too worried anymore:
https://seaborn.pydata.org/generated/seaborn.objects.Line.html
https://seaborn.pydata.org/generated/seaborn.objects.Lines.html

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Illviljan added 3 commits October 16, 2022 21:48

Add linecollection

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Illviljan and others added 24 commits October 18, 2022 20:01

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Illviljan commented Jul 21, 2025

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Illviljan and others added 14 commits July 21, 2025 23:24

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align with ax.scatter

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align more with ax.scatter

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extra linebreak helps?

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