Making var_group_rotation work when swap_axes is True

[gokceneraslan]

It'd be great if var_group_rotation option works also when swap_axes is True (right now rotation 270 is hard-coded). I tried to modify the code but I am not sure how to solve it properly (e.g. if labels are too long etc). Please have a look whenever you guys have time and feel free to modify the PR.

import scanpy as sc

ad = sc.datasets.paul15()
sc.pp.log1p(ad)
sc.tl.rank_genes_groups(ad, 'paul15_clusters')

sc.pl.rank_genes_groups_dotplot(ad, n_genes=3, dendrogram=False, var_group_rotation=0, groups=['1Ery', '2Ery', '3Ery'])
sc.pl.rank_genes_groups_dotplot(ad, n_genes=3, dendrogram=False, var_group_rotation=90, groups=['1Ery', '2Ery', '3Ery'])
sc.pl.rank_genes_groups_dotplot(ad, n_genes=3, dendrogram=False, swap_axes=True, var_group_rotation=0, groups=['1Ery', '2Ery', '3Ery'])
sc.pl.rank_genes_groups_dotplot(ad, n_genes=3, dendrogram=False, swap_axes=True, var_group_rotation=90, groups=['1Ery', '2Ery', '3Ery'])

Output:

[codecov]

Codecov Report

Merging #1763 (d882574) into master (560bd5d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1763   +/-   ##
=======================================
  Coverage   71.18%   71.19%           
=======================================
  Files          92       92           
  Lines       11190    11189    -1     
=======================================
  Hits         7966     7966           
+ Misses       3224     3223    -1     

Impacted Files	Coverage Δ
scanpy/plotting/_baseplot_class.py	84.70% <100.00%> (+0.23%)	⬆️