Point glyphs

Point Glyphs

To mimic the pch argument in R base graphics? e.g. pch=0 is square, 2 is circle, ...

There is a dilemma. We have two choices to draw points:

1. qdrawGlyph(): it does not accept vectorized glyphs (e.g. path = 1:10 won't work), but we can use the locate() method to identify graphical elements (e.g. points)
2. qdrawPath(): just opposite (maybe not exactly)

(Yihui: we can draw glyphs by groups using method 1 above, i.e. use loops to draw points; identifying elements in plots is a rather tedious programming job)