using signal in URL

signal_url.json

{
  "$schema": "https://vega.github.io/schema/vega/v4.json",
  "width": 400,
  "height": 200,
  "padding": 5,
  "title": "population trend",
  "signals": [
    {
      "name": "Species",
      "value": "Loxodonta cyclotis",
      "bind": {
        "input": "select",
        "options": [
          "Gorilla gorilla diehli",
          "Loxodonta africana",
          "Loxodonta cyclotis",
          "Mandrillus leucophaeus",
          "Osteolaemus tetraspis",
          "Ovibus moschatus",
          "Pan troglodytes",
          "Panthera leo",
          "Panthera onca",
          "Panthera pardus",
          "Panthera tigris altaica ",
          "Panthera tigris",
          "Picoides dorsalis",
          "Pteropus voeltzikowi"
        ]
      }
    },
    {
      "name": "Site",
      "value": "Lobeke NP",
      "bind": {
        "input": "select",
        "options": [
          "Azagny NP",
          "Bailly swamps",
          "Bangassou Forest Reserve",
          "Banyang-Mbo Forest Reserve",
          "Beki-boseemetie classified forest",
          "Boumba-Bek Forest Reserve",
          "Bwindi Impenetrable Forest NP",
          "Campo-Ma'an landscape",
          "Conkouati NP",
          "Dzanga Bai",
          "Dzanga-Sanga/Dzanga-Ndoki NP",
          "Garamba NP",
          "Ivindo NP plus its southern buffer",
          "Kabo",
          "Kahuzi Biega NP Lowland",
          "Kahuzi Biega NP Upland",
          "Korup NP",
          "Lac Tele",
          "Lobeke NP",
          "Lope NP",
          "Loundougou",
          "Luama-Itombwe West Area",
          "Maringa-Lopori area",
          "Mbam et Djerem",
          "Mengame WS",
          "Minkebe NP",
          "Mokabi",
          "Mongokele Forest Reserve",
          "Mt Cameroon",
          "Ngoila Mintom",
          "Ngombe concession and Ntokou Pikounda NP",
          "Nki Forest Reserve",
          "Nouabale-Ndoki NP",
          "Okapi NP",
          "Pokola",
          "Salonga NP",
          "Tai ecosystem"
        ]
      }
    },
    {
      "name": "address",
      "value": "https://library.wcs.org/DesktopModules/WCSVega/API/Data/PopulationMeasure?taxa=Loxodonta cyclotis&site=Lobeke NP",
      "update": "'https://library.wcs.org/DesktopModules/WCSVega/API/Data/PopulationMeasure?taxa=' + Species + '&site=' + Site"
    },
    {
      "name": "tooltip",
      "value": {},
      "on": [
        {
          "events": "rect:mouseover",
          "update": "datum"
        },
        {
          "events": "rect:mouseout",
          "update": "{}"
        }
      ]
    }
  ],
  "data": [
    {
      "name": "table",
      "url": {
        "signal": "address"
      },
      "format": {
        "type": "json",
        "parse": "auto"
      }
    }
  ],
  "scales": [
    {
      "name": "xscale",
      "type": "band",
      "domain": {
        "data": "table",
        "field": "surveyYear"
      },
      "range": "width",
      "padding": 0.05,
      "round": true
    },
    {
      "name": "yscale",
      "domain": {
        "data": "table",
        "field": "siteAbundance"
      },
      "nice": true,
      "range": "height"
    }
  ],
  "axes": [
    {
      "orient": "bottom",
      "scale": "xscale",
      "title": "Year"
    },
    {
      "orient": "left",
      "scale": "yscale",
      "title": "Number"
    }
  ],
  "marks": [
    {
      "type": "rect",
      "from": {
        "data": "table"
      },
      "encode": {
        "enter": {
          "x": {
            "scale": "xscale",
            "field": "surveyYear"
          },
          "width": {
            "scale": "xscale",
            "band": 1
          },
          "y": {
            "scale": "yscale",
            "field": "siteAbundance"
          },
          "y2": {
            "scale": "yscale",
            "value": 0
          }
        },
        "update": {
          "fill": {
            "value": "steelblue"
          }
        },
        "hover": {
          "fill": {
            "value": "grey"
          }
        }
      }
    },
    {
      "type": "text",
      "encode": {
        "enter": {
          "align": {
            "value": "center"
          },
          "baseline": {
            "value": "bottom"
          },
          "fill": {
            "value": "#333"
          }
        },
        "update": {
          "x": {
            "scale": "xscale",
            "signal": "tooltip.surveyYear",
            "band": 0.5
          },
          "y": {
            "scale": "yscale",
            "signal": "tooltip.siteAbundance",
            "offset": -2
          },
          "text": {
            "signal": "tooltip.siteAbundance"
          },
          "fillOpacity": [
            {
              "test": "datum === tooltip",
              "value": 0
            },
            {
              "value": 1
            }
          ]
        }
      }
    },
    {
      "type": "text",
      "interactive": false,
      "encode": {
        "enter": {
          "x": {
            "value": 895
          },
          "y": {
            "value": 0
          },
          "fill": {
            "value": "black"
          },
          "fontSize": {
            "value": 20
          },
          "align": {
            "value": "right"
          }
        },
        "update": {
          "text": {
            "signal": "address"
          }
        }
      }
    }
  ]
}