[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/javascripts/explorev2/stores/fields.js

@@ -72,6 +72,14 @@ export const fields = {
     description: 'Choose the metric',
   },
 
+  metric_2: {
+    type: 'SelectField',
+    label: 'Right Axis Metric',
+    choices: [],
+    default: [],
+    description: 'Choose a metric for right axis',
+  },
+
   stacked_style: {
     type: 'SelectField',
     label: 'Stacked Style',
@@ -692,6 +700,14 @@ export const fields = {
     description: D3_FORMAT_DOCS,
   },
 
+  y_axis_2_format: {
+    type: 'FreeFormSelectField',
+    label: 'Right axis format',
+    default: '.3s',
+    choices: D3_TIME_FORMAT_OPTIONS,
+    description: D3_FORMAT_DOCS,
+  },
+
   markup_type: {
     type: 'SelectField',
     label: 'Markup Type',