Document type hierarchy? about compose.jl HOT 1 OPEN

I started expanding the Compose docs in #303, and it's an ongoing process 😀.
To learn more, you can find examples of the outer constructors of those types in Gadfly.jl e.g. in Gadfly

• table is used by Geom.subplot_grid
• ctxpromise() is widely used in the text-related Guides to setup the possibility of horizontal or vertical text (if orientation=:auto), and in render_discrete_key() to setup a discrete key with an unknown number of columns.
• Later, Guide.layout_guides also uses table, which uses an optimization algorithm to decide among all the possibilities which one maximizes the plot space, under the constraint that the text (or key) needs to fit into the absolute plot area.

Point 3 is known as the "table layout problem".

from compose.jl.