How can the title of the mark group not be repeated when using facet? I believe the axes labels are also repeated and overlayed, although less noticeable.

Here’s an example:

The code to produce is based on here:

{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "description": "An interactive line chart of stock prices, with returns shown relative to a selected date.",
  "width": 650,
  "height": 300,
  "padding": 5,
  "autosize": {"type": "fit", "contains": "padding"},

  "signals": [
    {
      "name": "indexDate",
      "update": "time('Jan 1 2005')",
      "on": [
        {
          "events": "pointermove",
          "update": "invert('x', clamp(x(), 0, width))"
        }
      ]
    },
    {
      "name": "maxDate",
      "update": "time('Mar 1 2010')"
    }
  ],

  "data": [
    {
      "name": "stocks",
      "url": "data/stocks.csv",
      "format": {"type": "csv", "parse": {"price":"number", "date":"date"}}
    },
    {
      "name": "index",
      "source": "stocks",
      "transform": [
        {
          "type": "filter",
          "expr": "month(datum.date) == month(indexDate) && year(datum.date) == year(indexDate)"
        }
      ]
    },
    {
      "name": "indexed_stocks",
      "source": "stocks",
      "transform": [
        {
          "type": "lookup", "from": "index", "key": "symbol",
          "fields": ["symbol"], "as": ["index"], "default": {"price": 0}
        },
        {
          "type": "formula",
          "as": "indexed_price",
          "expr": "datum.index.price > 0 ? (datum.price - datum.index.price)/datum.index.price : 0"
        }
      ]
    }
  ],

  "scales": [
    {
      "name": "x",
      "type": "time",
      "domain": {"data": "stocks", "field": "date"},
      "range": "width"
    },
    {
      "name": "y",
      "type": "linear",
      "domain": {"data": "indexed_stocks", "field": "indexed_price"},
      "nice": true, "zero": true,
      "range": "height"
    },
    {
      "name": "color",
      "type": "ordinal",
      "range": "category",
      "domain": {"data": "stocks", "field": "symbol"}
    }
  ],

  "axes": [
    {"orient": "left", "scale": "y", "grid": true, "format": "%"}
  ],

  "marks": [
    {
      "type": "group",
      "title": {
        "text": "This is a test title"
      },
      "from": {
        "facet": {
          "name": "series",
          "data": "indexed_stocks",
          "groupby": "symbol"
        }
      },
      "data": [
        {
          "name": "label",
          "source": "series",
          "transform": [
            { "type": "filter", "expr": "datum.date == maxDate" }
          ]
        }
      ],
      "marks": [
        {
          "type": "line",
          "from": {"data": "series"},
          "encode": {
            "update": {
              "x": {"scale": "x", "field": "date"},
              "y": {"scale": "y", "field": "indexed_price"},
              "stroke": {"scale": "color", "field": "symbol"},
              "strokeWidth": {"value": 2}
            }
          }
        },
        {
          "type": "text",
          "from": {"data": "label"},
          "encode": {
            "update": {
              "x": {"scale": "x", "field": "date", "offset": 2},
              "y": {"scale": "y", "field": "indexed_price"},
              "fill": {"scale": "color", "field": "symbol"},
              "text": {"field": "symbol"},
              "baseline": {"value": "middle"}
            }
          }
        }
      ]
    },
    {
      "type": "rule",
      "encode": {
        "update": {
          "x": {"field": {"group": "x"}},
          "x2": {"field": {"group": "width"}},
          "y": {"value": 0.5, "offset": {"scale": "y", "value": 0, "round": true}},
          "stroke": {"value": "black"},
          "strokeWidth": {"value": 1}
        }
      }
    },
    {
      "type": "rule",
      "encode": {
        "update": {
          "x": {"scale": "x", "signal": "indexDate", "offset": 0.5},
          "y": {"value": 0},
          "y2": {"field": {"group": "height"}},
          "stroke": {"value": "firebrick"}
        }
      }
    },
    {
      "type": "text",
      "encode": {
        "update": {
          "x": {"scale": "x", "signal": "indexDate"},
          "y2": {"field": {"group": "height"}, "offset": 15},
          "align": {"value": "center"},
          "text": {"signal": "timeFormat(indexDate, '%b %Y')"},
          "fill": {"value": "firebrick"}
        }
      }
    }
  ]
}