[WIP] Add dual-axis line chart to viz (#1782)

* Add dual-axis line chart to viz
 - Add a new viz using two y axis for multi-chart in nvd3
 - Option to set metric and axis format for two y axis

* Resolve conflicts

* Fixed x axis and resized thumbnail

* Added example to __init__.py

* Change get_df to match with format

superset/assets/visualizations/main.js

@@ -29,5 +29,6 @@ const vizMap = {
   treemap: require('./treemap.js'),
   word_cloud: require('./word_cloud.js'),
   world_map: require('./world_map.js'),
+  dual_line: require('./nvd3_vis.js'),
 };
 export default vizMap;