I have two area marks in Vega (based on this), brushing the bottom chart zooms the top chart, and when zoomed in a lot it shows hours:

I’ve asked how to show days as the minimum unit here. Here I’d like to know how to set up the brush signal so that the brushed area snaps to multiples of a week. That way the top chart will only show a minimum of 7 days, then 14, etc.

{
  "$schema": "https://vega.github.io/schema/vega/v5.json",
  "description": "Area charts of stock prices, with an interactive overview and filtered detail views.",
  "width": 720,
  "height": 480,
  "padding": 5,
  "data": [
    {
      "name": "sp500",
      "url": "data/sp500.csv",
      "format": {"type": "csv", "parse": {"price": "number", "date": "date"}},
      "transform": [{"type": "filter", "expr": "datum.date < 957103200000"}]
    }
  ],
  "signals": [{"name": "detailDomain"}],
  "marks": [
    {
      "type": "group",
      "name": "detail",
      "encode": {"enter": {"width": {"value": 720}, "height": {"value": 390}}},
      "scales": [
        {
          "name": "xDetail",
          "type": "time",
          "range": "width",
          "domain": {"data": "sp500", "field": "date"},
          "domainRaw": {"signal": "detailDomain"}
        },
        {
          "name": "yDetail",
          "type": "linear",
          "range": [390, 0],
          "domain": {"data": "sp500", "field": "price"},
          "nice": true,
          "zero": true
        }
      ],
      "axes": [
        {"orient": "bottom", "scale": "xDetail"},
        {"orient": "left", "scale": "yDetail"}
      ],
      "marks": [
        {
          "type": "group",
          "encode": {
            "enter": {
              "height": {"field": {"group": "height"}},
              "width": {"field": {"group": "width"}},
              "clip": {"value": true}
            }
          },
          "marks": [
            {
              "type": "area",
              "from": {"data": "sp500"},
              "encode": {
                "update": {
                  "x": {"scale": "xDetail", "field": "date"},
                  "y": {"scale": "yDetail", "field": "price"},
                  "y2": {"scale": "yDetail", "value": 0},
                  "fill": {"value": "steelblue"}
                }
              }
            }
          ]
        }
      ]
    },
    {
      "type": "group",
      "name": "overview",
      "encode": {
        "enter": {
          "x": {"value": 0},
          "y": {"value": 430},
          "height": {"value": 70},
          "width": {"value": 720},
          "fill": {"value": "transparent"}
        }
      },
      "signals": [
        {
          "name": "brush",
          "value": 0,
          "on": [
            {"events": "@overview:pointerdown", "update": "[x(), x()]"},
            {
              "events": "[@overview:pointerdown, window:pointerup] > window:pointermove!",
              "update": "[brush[0], clamp(x(), 0, width)]"
            },
            {
              "events": {"signal": "delta"},
              "update": "clampRange([anchor[0] + delta, anchor[1] + delta], 0, width)"
            }
          ]
        },
        {
          "name": "anchor",
          "value": null,
          "on": [{"events": "@brush:pointerdown", "update": "slice(brush)"}]
        },
        {
          "name": "xdown",
          "value": 0,
          "on": [{"events": "@brush:pointerdown", "update": "x()"}]
        },
        {
          "name": "delta",
          "value": 0,
          "on": [
            {
              "events": "[@brush:pointerdown, window:pointerup] > window:pointermove!",
              "update": "x() - xdown"
            }
          ]
        },
        {
          "name": "detailDomain",
          "push": "outer",
          "on": [
            {
              "events": {"signal": "brush"},
              "update": "span(brush) ? invert('xOverview', brush) : null"
            }
          ]
        }
      ],
      "scales": [
        {
          "name": "xOverview",
          "type": "time",
          "range": "width",
          "domain": {"data": "sp500", "field": "date"}
        },
        {
          "name": "yOverview",
          "type": "linear",
          "range": [70, 0],
          "domain": {"data": "sp500", "field": "price"},
          "nice": true,
          "zero": true
        }
      ],
      "axes": [{"orient": "bottom", "scale": "xOverview"}],
      "marks": [
        {
          "type": "area",
          "interactive": false,
          "from": {"data": "sp500"},
          "encode": {
            "update": {
              "x": {"scale": "xOverview", "field": "date"},
              "y": {"scale": "yOverview", "field": "price"},
              "y2": {"scale": "yOverview", "value": 0},
              "fill": {"value": "steelblue"}
            }
          }
        },
        {
          "type": "rect",
          "name": "brush",
          "encode": {
            "enter": {
              "y": {"value": 0},
              "height": {"value": 70},
              "fill": {"value": "#333"},
              "fillOpacity": {"value": 0.2}
            },
            "update": {
              "x": {"signal": "brush[0]"},
              "x2": {"signal": "brush[1]"}
            }
          }
        },
        {
          "type": "rect",
          "interactive": false,
          "encode": {
            "enter": {
              "y": {"value": 0},
              "height": {"value": 70},
              "width": {"value": 1},
              "fill": {"value": "firebrick"}
            },
            "update": {"x": {"signal": "brush[0]"}}
          }
        },
        {
          "type": "rect",
          "interactive": false,
          "encode": {
            "enter": {
              "y": {"value": 0},
              "height": {"value": 70},
              "width": {"value": 1},
              "fill": {"value": "firebrick"}
            },
            "update": {"x": {"signal": "brush[1]"}}
          }
        }
      ]
    }
  ]
}