zorder.vl

{
    "config": {
        "axisY": {
            "gridColor": "blue"
        },
        "axisX": {
            "gridColor": "red"
        },
        "axis": {
            "gridWidth": 8
        }
    },
    "mark": {
        "size": 5000,
        "opacity": 1,
        "type": "circle"
    },
    "data": {
        "values": [
            {
                "cat": "a",
                "x": 20,
                "y": 10
            },
            {
                "cat": "b",
                "x": 10,
                "y": 20
            }
        ]
    },
    "$schema": "https://vega.github.io/schema/vega-lite/v3.json",
    "encoding": {
        "color": {
            "field": "cat",
            "type": "nominal",
            "legend": null
        },
        "x": {
            "field": "x",
            "type": "quantitative",
            "axis": {
                "zindex": 2
            }
        },
        "y": {
            "field": "y",
            "type": "quantitative",
            "axis": {
                "zindex": 1
            }
        }
    }
}