Use a default value for constant dimensions

[Illviljan]

• default markersize values of widths are 18 to 72.
• plt.scatter default markersize is 36.
• plt.plot default linewidth is 6.

With main we get 18 for constant arrays, but 36 if markersize was undefined. This seems a bit inconsistent to me.
This PR adds a default value instead for constant arrays

Follow up to #7272.

[Illviljan]

Markersize before PR:

Markersize after PR:

import numpy as np
import matplotlib.pyplot as plt

import xarray as xr

temp = xr.DataArray(np.random.randn(10, 10, 2), coords=[np.arange(10), np.arange(10), [2021, 2022]], dims=["lon", "lat", "year"])
prec = xr.DataArray(np.random.randn(10, 10, 2), coords=[np.arange(10), np.arange(10), [2021, 2022]], dims=["lon", "lat", "year"])
one = xr.DataArray(np.random.randn(10, 10, 2), coords=[np.arange(10), np.arange(10), [2021, 2022]], dims=["lon", "lat", "year"])
blue = xr.DataArray(np.random.randn(10, 10, 2), coords=[np.arange(10), np.arange(10), [2021, 2022]], dims=["lon", "lat", "year"])
ds = xr.Dataset({"temperature": temp, "precipitation": prec, 1: one, "blue": blue})

# Stack the non interesting dims:
# fig, ax = plt.subplots()
# ds.stack(stacked_dim=[...]).plot.scatter(x="temperature", y="precipitation")

fig, ax = plt.subplots()
ds.plot.scatter(x="temperature", y="precipitation", markersize=1)

[headtr1ck]

It's not really a range anymore, but since it is internal only I don't care much.