Fix type on same field name (GraphQL results)

api/src/engine/models/result/heat-map-result.model.ts

@@ -7,12 +7,12 @@ export class HeatMapResult extends Result {
   @Field()
   name: string;
 
-  @Field(() => [[Number]])
-  matrix: number[][];
+  @Field(() => ChartAxis, { nullable: true })
+  xAxis?: ChartAxis;
 
-  @Field(() => ChartAxis)
-  xAxis: ChartAxis;
+  @Field(() => ChartAxis, { nullable: true })
+  yAxis?: ChartAxis;
 
-  @Field(() => ChartAxis)
-  yAxis: ChartAxis;
+  @Field(() => [[Number]])
+  matrix: number[][];
 }