Document matplotlib dependency/ install matplotlib in CI #13335
Description
We've introduced matplotlib dependency in some functions, e.g., plot_tree
(1) We need to document the min version we require (the minimal version of matplotlib for Ubuntu 16.04 is 1.5.1). Maybe also document the classes/functions which depend on matplotlib
(2) We need to install matplotlib in CIs
(3) Maybe we can remove some redundant code, e.g.,
scikit-learn/sklearn/tree/export.py
Lines 544 to 546 in 3f0c5fe