"labels" on bars

vega.json

{
    "width": 400,
    "height": 400,
    "padding": {"top": 10, "left": 30, "bottom": 20, "right": 10},
    "data": [
        {
            "name": "table",
            "values": [
                {"category":"A", "amount":28},
                {"category":"B", "amount":55},
                {"category":"C", "amount":43},
                {"category":"D", "amount":91},
                {"category":"E", "amount":81},
                {"category":"F", "amount":53},
                {"category":"G", "amount":19},
                {"category":"H", "amount":87},
                {"category":"I", "amount":52}
            ]
        }
    ],
    "scales": [
        {
            "name": "xscale",
            "type": "ordinal",
            "range": "width",
            "domain": {
                "data": "table",
                "field": "category"
            }
        },
        {
            "name": "yscale",
            "range": "height",
            "nice": true,
            "domain": {
                "data": "table",
                "field": "amount"
            }
        }
    ],
    "axes": [
        {
            "type": "x",
            "scale": "xscale"
        },
        {
            "type": "y",
            "scale": "yscale"
        }
    ],
    "marks": [
        {
            "type": "rect",
            "from": {
                "data":"table"
            },
            "properties": {
                "update": {
                    "fill": {
                        "value": "steelblue"
                    },
                    "x": {
                        "scale": "xscale",
                        "field": "category"
                    },
                    "width": {
                        "scale": "xscale",
                        "band": true,
                        "offset": -1
                    },
                    "y": {
                        "scale": "yscale",
                        "field": "amount"
                    },
                    "y2": {
                        "scale": "yscale",
                        "value": 0
                    }
                }
            }
        },
        {
            "type": "text",
            "from": {
                "data":"table"
            },
            "properties": {
                "update": {
                    "fill": {
                        "value": "#333"
                    },
                    "align": {
                        "value": "center"
                    },
                    "x": {
                        "scale": "xscale",
                        "field": "category"
                    },
                    "dx": {
                        "scale": "xscale",
                        "band": true,
                        "mult": 0.5
                    },
                    "y": {
                        "scale": "yscale",
                        "field": "amount",
                        "offset": -5
                    },
                    "text": {
                        "field": "amount"
                    }
                }
            }
        }
    ]
}