score:1

Accepted answer

It's not advised to set a domain using static values. Here's a similar example for what you're looking for:

https://bl.ocks.org/mbostock/3887118

Using the following changes:

  1. Defining width, height, margins as variables.

    var margin = {top: 10, bottom: 10, left: 10, right: 10},
      width = 600-margin.left-margin.right,
      height = 400-margin.top-margin.bottom;
    
  2. Reason for the linear scale returning NaN is becuase the data values (x & y) are strings and not numbers. To parse the data, here's one approach:

    // parse data
    myData.forEach(function(d) {
     d.x = +d.x;
     d.y = +d.y;
    });
    
  3. Important advise: Avoid using static values while setting the axes domains. Make use of d3 array manipulations-min,max,extent. I'm using d3-extent

    var xline = d3.scaleLinear().domain(d3.extent(myData, function(d) { return d.x; })).nice().range([0, width]);
    var yline = d3.scaleLinear().domain(d3.extent(myData, function(d) { return d.y; })).nice().range([height, 0]);
    
  4. As the data is now parsed (2nd step), linear scale wouldn't have issue plotting the values.

    var circleElements = svgViewport.selectAll("circle").data(myData).enter().append("circle").attr("cx", function(d) {
      return xline(d.x);
    }).attr("cy", function(d) {
      return yline(d.y);
    })
    

Just for aesthetics, I'm using a color scale to add colors to the circles:

var colorScale = d3.scaleOrdinal(d3.schemeCategory10);

Here's a code snippet:

  var myData = [{
    "x": "45.4969717",
    "y": "10.605375399999957"
  }, {
    "x": "45.4847328802866",
    "y": "9.236798286437988"
  }, {
    "x": "42.4838657",
    "y": "9.25125330000003",
  }, {
    "x": "40.1053737",
    "y": "12"
  }, {
  	"x": "42.4",
    "y": "10.4"
  }];
  
  var margin = {top: 10, bottom: 10, left: 10, right: 10},
 		  width = 450.57913-margin.left-margin.right,
  		height = 350-margin.top-margin.bottom;
  
  
// parse data
myData.forEach(function(d) {
	d.x = +d.x;
  d.y = +d.y;
});

  var xline = d3.scaleLinear().domain(d3.extent(myData, function(d) { return d.x; })).nice().range([0, width]);
  var yline = d3.scaleLinear().domain(d3.extent(myData, function(d) { return d.y; })).nice().range([height, 0]);
  
  var colorScale = d3.scaleOrdinal(d3.schemeCategory10);
  
  var svgViewport = d3.select("#mappa-italia")
  			.attr('width', width+margin.left+margin.right)
        .attr('height', height+margin.top+margin.bottom)
        .append("g")
        .attr('transform', 'translate('+margin.left+', ' + margin.top+')')
        .attr("id", "locations");
  var circleElements = svgViewport.selectAll("circle").data(myData).enter().append("circle").attr("cx", function(d) {
    return xline(d.x);
  }).attr("cy", function(d) {
    return yline(d.y);
  }).attr("r", "5").style('fill', function(d, i) {
		return colorScale(i);
  });
<script src="https://d3js.org/d3.v5.min.js"></script>


<svg id="mappa-italia"></svg>

Hope this helps. (Btw I've added a couple of extra rows in the data)

score:2

The domain needs to be an array so:

var xline = d3.scaleLinear().domain([45.103300,40.1053737]).range([0,450.57913]);
var yline = d3.scaleLinear().domain([12.186311,12.605783]).range([0,350]);

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