feat: add band area chart (#157)

This PR adds a `y0Accessors` prop for each `SeriesSpec`: this property can be used to describe a dataset with bands ( with a min and max value for each data point).
It's mainly used for with `AreaSeries` to describe a bounded area chart, bounded on the y axis. The  `y0` value, is usually a lower value than the `y1`, it will be rendered perpendicularly to the `y1`  point. On a standard area chart, the `y0` value usually correspond to `0` or to the previous value in the stack. In a band-area chart, the `y0` can be any value below the `y1` value.
It can also be applied to bar charts, with some limitations as described below. On a barchart, the `y0` value is the bottom edge of the bar.
On a stacked bar/area chart a series with a `y0Accessor` will be stacked on top of the below series stacking the below series `y1` value with the band series `y0` value. This means that if the below series at a point X has an height of `10`, and the bound series has the following values: `y0: 2, y1: 5` the resulting chart will push the bound series to the top starting using the following values: `y0:12, y1: 15`. The `IndexedGeometry` object is also refactored. To limit amount of redundant code/memory object we used the same `Geometry` elements used on the highlight/hover process.
The structure of `IndexedGeometry` is slightly changed, to avoid duplicate values and to simplify tooltip formatting when rendering either y1 and y0 values.  
I've currently removed from the `indexedGeometries` map all geometry with `null` values on `y1`. This leads to a minor problem: we are not showing any tooltip on that interval, the tooltip of the crosshair is never shown.

close #144

src/components/highlighter.tsx

@@ -1,5 +1,6 @@
 import { inject, observer } from 'mobx-react';
 import React from 'react';
+import { isPointGeometry } from '../lib/series/rendering';
 import { ChartStore } from '../state/chart_state';
 
 interface HighlighterProps {
@@ -21,26 +22,31 @@ class HighlighterComponent extends React.Component<HighlighterProps> {
     return (
       <svg className="elasticChartsHighlighter">
         <g transform={`translate(${left}, ${top}) rotate(${chartRotation})`}>
-          {highlightedGeometries.map((highlightedGeometry, i) => {
-            const {
-              color,
-              geom: { x, y, width, height, isPoint },
-            } = highlightedGeometry;
-            if (isPoint) {
+          {highlightedGeometries.map((geom, i) => {
+            const { color, x, y } = geom;
+            if (isPointGeometry(geom)) {
               return (
                 <circle
                   key={i}
-                  cx={x}
+                  cx={x + geom.transform.x}
                   cy={y}
-                  r={width}
+                  r={geom.radius}
                   stroke={color}
                   strokeWidth={4}
                   fill="transparent"
                 />
               );
             }
             return (
-              <rect key={i} x={x} y={y} width={width} height={height} fill="white" opacity={0.4} />
+              <rect
+                key={i}
+                x={x}
+                y={y}
+                width={geom.width}
+                height={geom.height}
+                fill="white"
+                opacity={0.4}
+              />
             );
           })}
         </g>

src/components/react_canvas/area_geometries.tsx

@@ -42,7 +42,7 @@ export class AreaGeometries extends React.PureComponent<
     return (
       <Group ref={this.barSeriesRef} key={'bar_series'}>
         {area.visible && this.renderAreaGeoms()}
-        {line.visible && this.renderAreaLine()}
+        {line.visible && this.renderAreaLines()}
         {point.visible && this.renderAreaPoints()}
       </Group>
     );
@@ -132,46 +132,51 @@ export class AreaGeometries extends React.PureComponent<
       }
     });
   }
-  private renderAreaLine = (): JSX.Element[] => {
+  private renderAreaLines = (): JSX.Element[] => {
     const { areas, sharedStyle } = this.props;
     const { strokeWidth } = this.props.style.line;
-
-    return areas.map((glyph, i) => {
-      const { line, color, transform, geometryId } = glyph;
+    const linesToRender: JSX.Element[] = [];
+    areas.forEach((glyph, areaIndex) => {
+      const { lines, color, geometryId } = glyph;
 
       const geometryStyle = getGeometryStyle(
         geometryId,
         this.props.highlightedLegendItem,
         sharedStyle,
       );
 
-      if (this.props.animated) {
-        return (
-          <Group key={`area-line-group-${i}`} x={transform.x}>
-            <Spring native from={{ line }} to={{ line }}>
-              {(props: { line: string }) => {
-                const lineProps = buildAreaLineProps({
-                  index: i,
-                  linePath: props.line,
-                  color,
-                  strokeWidth,
-                  geometryStyle,
-                });
-                return <animated.Path {...lineProps} />;
-              }}
-            </Spring>
-          </Group>
-        );
-      } else {
+      lines.forEach((linePath, lineIndex) => {
         const lineProps = buildAreaLineProps({
-          index: i,
-          linePath: line,
+          areaIndex,
+          lineIndex,
+          linePath,
           color,
           strokeWidth,
           geometryStyle,
         });
-        return <Path {...lineProps} />;
-      }
+        linesToRender.push(<Path {...lineProps} />);
+      });
     });
+    return linesToRender;
+    // if (this.props.animated) {
+    //   return (
+    //     <Group key={`area-line-group-${i}`} x={transform.x}>
+    //       <Spring native from={{ line }} to={{ line }}>
+    //         {(props: { line: string }) => {
+    //           const lineProps = buildAreaLineProps({
+    //             index: i,
+    //             linePath: props.line,
+    //             color,
+    //             strokeWidth,
+    //             geometryStyle,
+    //           });
+    //           return <animated.Path {...lineProps} />;
+    //         }}
+    //       </Spring>
+    //     </Group>
+    //   );
+    // } else {
+
+    // }
   }
 }

src/components/react_canvas/utils/rendering_props_utils.test.ts

@@ -80,7 +80,8 @@ describe('[canvas] Area Geometries props', () => {
   });
   test('can build area line path props', () => {
     const props = buildAreaLineProps({
-      index: 1,
+      areaIndex: 1,
+      lineIndex: 2,
       linePath: 'M0,0L10,10Z',
       color: 'red',
       strokeWidth: 1,
@@ -89,7 +90,7 @@ describe('[canvas] Area Geometries props', () => {
       },
     });
     expect(props).toEqual({
-      key: `area-line-1`,
+      key: `area-1-line-2`,
       data: 'M0,0L10,10Z',
       stroke: 'red',
       strokeWidth: 1,

src/components/react_canvas/utils/rendering_props_utils.ts

@@ -57,20 +57,22 @@ export function buildAreaProps({
 }
 
 export function buildAreaLineProps({
-  index,
+  areaIndex,
+  lineIndex,
   linePath,
   color,
   strokeWidth,
   geometryStyle,
 }: {
-  index: number;
+  areaIndex: number;
+  lineIndex: number;
   linePath: string;
   color: string;
   strokeWidth: number;
   geometryStyle: GeometryStyle;
 }) {
   return {
-    key: `area-line-${index}`,
+    key: `area-${areaIndex}-line-${lineIndex}`,
     data: linePath,
     stroke: color,
     strokeWidth,