refine line.
refine scatter.
refine bar.

and merged bar and stacked bar.
refine area.

and merged area and stacked area.
refined grouped bar chart.
final pep8 cleanup for map test.

add tests to flake8 check now.
Merge pull request #86 from lxyu/master

marks always have a type
Merge pull request #87 from lxyu/refine_charts

Refine charts