View Concatenation

The vconcat and hconcat composition operators place views side-by-side either vertically or horizontally. The vconcat is practical for building genomic visualizations with multiple tracks. The concat operator with the columns property produces a wrapping grid layout.

The spacing (in pixels) between concatenated views can be adjusted using the spacing property (Default: 10).

Example

Vertical

Using vconcat for a vertical layout.

{
  "data": { "url": "sincos.csv" },

  "spacing": 20,

  "vconcat": [
    {
      "mark": "point",
      "encoding": {
        "x": { "field": "x", "type": "quantitative" },
        "y": { "field": "sin", "type": "quantitative" }
      }
    },
    {
      "mark": "point",
      "encoding": {
        "x": { "field": "x", "type": "quantitative" },
        "y": { "field": "cos", "type": "quantitative" }
      }
    }
  ]
}

Horizontal

Using hconcat for a horizontal layout.

{
  "data": { "url": "sincos.csv" },

  "hconcat": [
    {
      "mark": "point",
      "encoding": {
        "x": { "field": "x", "type": "quantitative" },
        "y": { "field": "sin", "type": "quantitative" }
      }
    },
    {
      "mark": "point",
      "encoding": {
        "x": { "field": "x", "type": "quantitative" },
        "y": { "field": "cos", "type": "quantitative" }
      }
    }
  ]
}

Grid

Using concat and columns for a grid layout. For simplicity, the same visualization is used for all panels in the grid.

{
  "data": { "url": "sincos.csv" },
  "encoding": {
    "x": { "field": "x", "type": "quantitative" },
    "y": { "field": "sin", "type": "quantitative" }
  },

  "columns": 3,
  "concat": [
    { "mark": "point" },
    { "mark": "point" },
    { "mark": "point" },
    { "mark": "point" },
    { "mark": "point" },
    { "mark": "point" },
    { "mark": "point" },
    { "mark": "point" },
    { "mark": "point" }
  ]
}

Child sizing

The concatenation operators mimic the behavior of the CSS flexbox. The child views have an absolute minimum size (px) in pixels and an unitless grow value that specifies in what proportion the possible remaining space should be distributed. The remaining space depends on the parent view's size.

In the following example, the left view has a width of 20 px, the center view has a grow of 1, and the right view has a grow of 2. If you resize the web browser, you can observe that the width of the left view stays constant while the remaining space is distributed in proportions of 1:2.

{
  "data": { "values": [{}] },

  "spacing": 10,

  "hconcat": [
    {
      "width": { "px": 20 },
      "mark": "rect"
    },
    {
      "width": { "grow": 1 },
      "mark": "rect"
    },
    {
      "width": { "grow": 2 },
      "mark": "rect"
    }
  ]
}

The size may have both absolute (px) and proportional (grow) components. When views are nested, both the absolute and proportional sizes are added up. Thus, the width of the above example is { "px": 40, "grow": 3 }. The spacing between the child views is added to the total absolute width.

Views' size properties (width and height) accept both SizeDef objects and shorthands. The SizeDef objects contain either or both of px and grow properties. Numbers are interpreted as as absolute sizes, and "container" is the same as { grow: 1 }. Undefined sizes generally default to "container".

Concatenation operators can nested flexibly to build complex layouts as in the following example.

{
  "data": { "values": [{}] },

  "hconcat": [
    { "mark": "rect" },
    {
      "vconcat": [{ "mark": "rect" }, { "mark": "rect" }]
    }
  ]
}

Resolve

By default, all channels have independent scales and axes. However, because track-based layouts that resemble genome browsers are such a common use case, vconcat has a special treatment for the x channel: if all child views use "locus" or "index" data type on the x channel, the scale resolution defaults to "shared". However, axes are still "independent" by default.

Shared axes

Concatenation operators support shared axes on channels that also have shared scales. Axis domain line, ticks, and labels are drawn only once for each row or column. Grid lines are drawn for all participating views.

{
  "data": { "url": "sincos.csv" },

  "resolve": {
    "scale": { "x": "shared", "y": "shared" },
    "axis": { "x": "shared", "y": "shared" }
  },

  "spacing": 20,

  "encoding": {
    "x": { "field": "x", "type": "quantitative", "axis": { "grid": true } },
    "y": { "field": "sin", "type": "quantitative", "axis": { "grid": true } }
  },

  "columns": 2,

  "concat": [
    { "mark": "point", "view": { "stroke": "lightgray" } },
    { "mark": "point", "view": { "stroke": "lightgray" } },
    { "mark": "point", "view": { "stroke": "lightgray" } },
    { "mark": "point", "view": { "stroke": "lightgray" } }
  ]
}