REQ: vega-cli (related to py36-vega and py36-altair)

[jowens (John Owens)]

The "vega" visualization grammar is used in altair; relevant python36 packages are py36-vega and py36-altair. It appears that vega and vega-lite install as part of altair (it does not depend on py36-vega). However, neither appears to install the Vega command line package vega-cli:

​https://vega.github.io/vega/usage/#cli

It appears to use node-canvas. The easy-to-follow directions of yarn global add vega-cli or npm install -g vega-cli did not work with my MacPorts install, and this is not a build environment with which I am familiar (happy to provide debug info, but someone who knows this stuff is better qualified).

I'd love to see this port (vega-cli) added to MacPorts.