feat: update line chart thumbnail and scale extraction (#61)

* feat: update line chart thumbnail

* fix: lint

plugins/superset-ui-plugins/packages/superset-ui-preset-chart-xy/src/Line/Encoder.ts

@@ -13,13 +13,13 @@ import { EncodingFromChannelsAndOutputs } from '../encodeable/types/Channel';
 // See more details from
 // https://github.com/Microsoft/TypeScript/issues/20195
 // https://github.com/Microsoft/TypeScript/pull/29510
-const channelTypes = new class Channels {
+const channelTypes = new (class Channels {
   readonly x = 'X';
   readonly y = 'Y';
   readonly color = 'Color';
   readonly fill = 'Category';
   readonly strokeDasharray = 'Category';
-}();
+})();
 
 export type ChannelTypes = typeof channelTypes;
 

plugins/superset-ui-plugins/packages/superset-ui-preset-chart-xy/src/encodeable/parsers/extractScale.ts

@@ -77,9 +77,13 @@ export function deriveScaleTypeFromDataTypeAndChannelType(
     return undefined;
   } else if (dataType === 'nominal' || dataType === 'ordinal') {
     switch (channelType) {
+      // For positional (x and y) ordinal and ordinal fields,
+      // "point" is the default scale type for all marks
+      // except bar and rect marks, which use "band" scales.
+      // https://vega.github.io/vega-lite/docs/scale.html
       case 'XBand':
       case 'YBand':
-        return ScaleType.POINT;
+        return ScaleType.BAND;
       case 'X':
       case 'Y':
       case 'Numeric':