`fromto` trace type proposal

I’d like to propose a new trace type, provisionally called fromto, which would be inspired partly by ggplot2’s geom_segment (paging @cpsievert!)

The behaviour would be to draw onto cartesian axes a line segment from x/ y to xend/yend (all 4 being data arrays) with the ability to specify what the start/end points look like: arrows, points etc. More advanced variations might include curved lines, or manhattan lines or whatever. We would be able to reuse a lot of pre-existing machinery to do this: lines, points, arrows from annotations etc 😃

The use-cases for such a trace-type are multiple:

1. Gantt charts (both for the time-bars and for the dependencies!)
2. Connected dot-plots
3. Quiver plots
4. The ‘link’ portion of a node-link diagram

[johentsch]

IMHO this feature would make Plotly the greatest plotting library of all!

It has been mentioned in several places (e.g. in #147) that it is needed for professionally plotting weighted and potentially directed network plots. Not only do you want to set direction, width and color of edges (which you can awkwardly do with annotations), you also want them to be data themselves with their own hover information giving information about weights and (in the case I am currently working on) previous nodes included in the path.

[nicolaskruchten]

Makes sense, thanks for expanding!

So I think this trace type still makes sense even if it’s just mainly aimed at connected-dot-plots of the type you made above or the more boring shared-x case. And if we build this, we give nicer JS-level (i.e. Chart Studio-level!!) support for building simple, non-automated Gantt charts, and we can refactor stuff like the figure factory (which itself doesn’t today support dependency and subtask modelling, right?), and this is a ‘building block’ trace type that can be used to approximate all sorts of other things like quiver plots, until such time as we can build out “proper” ones 😃