                        }]
                    },
                    test_a2: {
                        desc: 'matrix.x.data is empty array, expect no error.',
                        data: {
                            x: [],
                            y: ['a', 'X2', null, 'd'],
                        },
                    },
                    test_b: {
                        desc: '[null] is allowed; number is illegal in matrix.x/y.data and replaced by auto-gen text.',
                        data: