a component of #3122
add entities in to line plot
perf note. I noticed that plot's .entities method does a couple linear time passes on the data. Notably, it uses _lightweightEntities then maps them to "heavyweight" entities, which as far as i can tell, just adds a d3 selection. For perf, we should maybe consider using lightweight entities when filtering, then convert to PlotEntities as a post process. Lots of other methods use this.entities() so, probably not worth in this PR, but for the future.
Filed as #3145
Looks good. Keep em coming'
heh, no brainer
nit: should we declare an abstract entitiesIn method on XYPlot?
nit: we use this Range type to specify a range of numbers, but I've also seen [number, number] in some places