版本：5.21.0
- https://cdn.bootcdn.net/ajax/libs/vega/5.21.0/vega-core.js
- https://cdn.bootcdn.net/ajax/libs/vega/5.21.0/vega-core.min.js
- https://cdn.bootcdn.net/ajax/libs/vega/5.21.0/vega.js
- https://cdn.bootcdn.net/ajax/libs/vega/5.21.0/vega.min.js
The Vega visualization grammar.