Add magrittr dependency
First changes for new syntax
Don't explicitly store data_id or props_id
Rename 'chidren' to 'layers'
Modifications to as.vega to make basic plots work
Get transform_sort working
Fix props ID calculation
layer_point: simplify data ID code
Add visualize alias
Add register_data function
Migrate all marks to new format
Reorganize code
Add explain() function
Rename visualise to ggvis
Rename layer to mark
Move transform_bin to new system
Small cleanup for consistency
Add branch() and defer()
Convert layer_histogram to use branch
defer: fix support for anonymous functions