GitLab

* fixed description of Dataset

* fixed description of Booster

* fixed description of train

* fixed description of cv

* fixed description of callbacks

* fixed description of plotting

* hotfixes

* Fix Invalid decision type error while plotting tree in Python

* Fix Invalid decision type error while plotting tree in Python (modified = to ==)

* python-package: add graphviz.Digraph parameters

* examples: add a plottig example with graphviz

* fix tree index in print

* python-package: refine plot_tree

This change splits plot_tree to two methods:
1. method of creating digraph of tree
2. method of plotting the digraph with matplotlib

* fix doc

* fix doc

* refine plot

* use warnings

* refine  logic

* revert 'move to compat.py'

* add plot metrics

* move 'raise Exception' to check_not_tuple_of_2_elements

* rename 'plot_metrics' to 'plot_metric'

* fix misleading message/docs

* change 'Metrics' in title to 'Metric'

* fix misleading comment

* add plot tree

* add docs

* add example

* add test

* fix test

* fix decision type

* add show_info

* use feature name if available

* add plot importance

* add plot example