feat(matrix): Support matrix.x/y.length for conveniently create a headless matrix without composing an array.

src/coord/matrix/MatrixDim.ts

@@ -149,6 +149,7 @@ export class MatrixDim {
         this._uniqueValueGen = createUniqueValueGenerator(dim);
 
         let dimModelData = dimModel.get('data', true);
+        const length = dimModel.get('length', true);
         if (dimModelData != null && !isArray(dimModelData)) {
             if (__DEV__) {
                 error(`Illegal echarts option - matrix.${this.dim}.data must be an array if specified.`);
@@ -158,6 +159,13 @@ export class MatrixDim {
         if (dimModelData) {
             this._initByDimModelData(dimModelData);
         }
+        else if (length != null) {
+            dimModelData = Array(length);
+            for (let i = 0; i < length; i++) {
+                dimModelData[i] = null;
+            }
+            this._initByDimModelData(dimModelData);
+        }
         else {
             this._initBySeriesData();
         }

src/coord/matrix/MatrixModel.ts

@@ -158,6 +158,9 @@ interface MatrixDimensionOption extends MatrixCellStyleOption, MatrixDimensionLe
     type?: 'category'; // For internal usage; force be 'category'.
     show?: boolean;
     data?: MatrixDimensionCellLooseOption[];
+    // A simple way to provide column/row count if no need to compose a `data`.
+    // Note: `length` is ignored if `data` is specified.
+    length?: number;
     // `levels[0]`: the topmost (for x dimension) or leftmost (for y dimension) level.
     // If not specified, use null/undefined, such as `levels: [null, null, {levelSize: 10}]`
     levels?: (MatrixDimensionLevelOption | NullUndefined)[];