file.json

{
    $schema: https://vega.github.io/schema/vega/v5.json
    config: {
      kibana: {
        restoreSignalValuesOnRefresh: true
        controlsLocation: right
      }
    }
    data: [
      {
        name: data
        values: [
          {
            first_name: john
            middle_name: cleese
            last_name: jones
          }
          {
            first_name: brian
            middle_name: james
            last_name: jones
          }
          {
            first_name: john
            middle_name: mark
            last_name: tim
          }
          {
            first_name: tom
            middle_name: long middle name
            last_name: tim
          }
          {
            first_name: adam
            middle_name: ewan
            last_name: long surname
          }
        ]
        transform: [
          {
            type: formula
            expr: if(selector=="first_name", datum.first_name, if(selector=="middle_name", datum.middle_name, if(selector=="last_name", datum.last_name, false)))
            as: var
          }
          {
            type: aggregate
            groupby: [
              var
            ]
            ops: [
              count
            ]
            as: [
              doc_count
            ]
          }
        ]
      }
    ]
    signals: [
      {
        name: selector
        value: first_name
        bind: {
          input: select
          options: [
            first_name
            middle_name
            last_name
          ]
        }
      }
      {
        name: width
        update: width-120
      }
    ]
    scales: [
      {
        name: x
        type: band
        range: height
        padding: 0.05
        domain: {
          data: data
          field: var
        }
      }
      {
        name: y
        type: linear
        range: width
        domain: {
          data: data
          field: doc_count
        }
      }
    ]
    axes: [
      {
        orient: left
        scale: x
      }
      {
        orient: bottom
        scale: y
      }
    ]
    marks: [
      {
        type: rect
        from: {
          data: data
        }
        encode: {
          update: {
            y: {
              field: var
              scale: x
            }
            x: {
              field: doc_count
              scale: y
            }
            height: {
              scale: x
              band: 1
            }
            x2: {
              scale: y
              value: 0
            }
          }
        }
      }
    ]
  }