Visualization for multiple line charts (#4819)

* Initial test

* Save

* Working version

* Use since/until from payload

* Option to prefix metric name

* Rename LineMultiLayer to MultiLineViz

* Add more styles

* Explicit nulls

* Add more x controls

* Refactor to reuse nvd3_vis

* Fix x ticks

* Fix spacing

* Fix for druid datasource

* Rename file

* Small fixes and cleanup

* Fix margins

* Add proper thumbnails

* Align yaxis1 and yaxis2 ticks

* Improve code

* Trigger tests

* Move file

* Small fixes plus example

* Fix unit test

* Remove SQL and Filter sections

superset/assets/src/explore/components/controls/DatasourceControl.jsx

@@ -18,7 +18,7 @@ const propTypes = {
   name: PropTypes.string.isRequired,
   onChange: PropTypes.func,
   value: PropTypes.string.isRequired,
-  datasource: PropTypes.object.isRequired,
+  datasource: PropTypes.object,
 };
 
 const defaultProps = {