Distinguish between mark_point, mark_circle, and mark_square

[kdorr]

mpl-altair does not distinguish between mark_point(), mark_circle(), and mark_square(). All three marks produce a scatter plot.

Implementation Notes

• Marks should be handled in _marks.py
• mark_point() should draw open circles by default
• mark_circle() should draw closed circles by default
• mark_square() should draw squares by default

For more information

• Altair user_guide