Order of cell plotting differs in UMAP and scatterplot

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  Scanpy version:1.8.1
  May be related to: https://github.com/theislab/scanpy/issues/1263

In some scatterplot-based plots it is possible to set custom cell order (e.g. UMAP), while it others it is not possible (e.g. scatterplot). Solution: preserve cell order in scatterplot when possible.

Reproduce:
Want the class 2 not to be on class 1 - works on UMAp but not on scatterplot.

import scanpy as sc
import pandas as pd
import numpy as np

x=np.random.normal(size=200)
y=np.random.normal(size=200)
adata=sc.AnnData(obs=pd.DataFrame({'color':['1']*100+['2']*100,'x':x,'y':y}))
adata.obsm['X_umap']=np.array([x,y]).T

Scatterplot, default order

sc.pl.scatter(adata, x='x', y='y', color='color') 

Scatterplot, try to set random order, not working

np.random.seed(0)
random_indices=np.random.permutation(list(range(adata.shape[0])))
sc.pl.scatter(adata[random_indices,:], x='x', y='y',   color='color') 

UMAP, default order

sc.pl.umap(adata,  color='color') 

UMAP, try to set random order, works

np.random.seed(0)
random_indices=np.random.permutation(list(range(adata.shape[0])))
sc.pl.umap(adata[random_indices,:],   color='color') 

Start with the sc.pl.scatter and sc.pl.umap entry points and run the reproduction using the supplied AnnData and shuffled indices. Compare how each plot handles cell ordering, then verify that scatterplot preserves the supplied order when possible and matches the demonstrated UMAP behavior.