state: new `getYScaleByOrientation` function

src/models/series.ts

@@ -35,6 +35,7 @@ export class CoreSeries<T extends Serie> {
   _series: Array<T> = [];
 
   constructor(series: T[], private _podDefaults?: Partial<T>) {
+    // TODO: create separate Serie class, and store instances in this._series
     this.setSeries(series);
   }
 

src/models/state.ts

@@ -1,4 +1,4 @@
-import { Serie, Options } from '../types';
+import { Serie, Options, yAxisOrientation } from '../types';
 import { CoreSeries } from './series';
 import { CoreOptions } from './options';
 
@@ -89,6 +89,15 @@ export class PodState<T extends Serie, O extends Options> {
     this._transform = { x: 0, y: 0, k: 1 };
   }
 
+  getYScaleByOrientation(orientation?: yAxisOrientation): d3.ScaleLinear<number, number> {
+    // TODO: we set defaults in Series class, so we don't expect `undefined` here
+    // we can remove this check when we implement Serie class (see TODO in `series.ts`)
+    if(orientation === undefined) {
+      return this._yScale;
+    }
+    return orientation === yAxisOrientation.LEFT ? this._yScale : this._y1Scale;
+  }
+
   get yScale(): d3.ScaleLinear<number, number> {
     return this._yScale;
   }