force sim. along one axis w. bounds

<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <script src="https://d3js.org/d3.v4.min.js"></script>
  <style>
    body { margin:0;position:fixed;top:0;right:0;bottom:0;left:0; }
    body { padding: 50px}
    svg { border: 1px solid #ff3333;}
  </style>
</head>
   
<body>
  <script>
    const height = 350,
          width = 300,
          dotradius = 20;
    
    // Feel free to change or delete any of the code you see in this editor!
    var svg = d3.select("body").append("svg")
      .attr("width", width)
      .attr("height", height)
                           
    const data = [
    	{ y: 30, x: width/2},  
    	{ y: 30, x: width/2},  
    	{ y: 30, x: width/2},  
    	{ y: 30, x: width/2},  
    	{ y: 30, x: width/2},  
    	{ y: 30, x: width/2},  
    	{ y: 30, x: width/2},  
    ];                            
                      
     svg.append("g").classed("pre", true).selectAll("circle")
    	.data(data)
    	.enter()
    		.append("circle") 
    		.attr("cx", d => d.x)
    		.attr("cy", d => d.y)
    		.attr("r", dotradius)
     		.style("fill", "black")       
    		.style("fill-opacity",0.2)
     		.style("stroke", "black")
    		.style("stroke-opacity",0.5)
                                                                                                                                                                                  
    const force = d3.forceSimulation()               
      .nodes(data)                                  
    	.force('forceManyBody', d3.forceManyBody().strength(-1000))
      //.force('collide', d3.forceCollide(dotradius).strength(1))
      //.force('x', d3.forceX().strength(0.1))
      .force('axis', box_force)
    	.stop();
                  
    function box_force() { 
      for (var i = 0, n = data.length; i < n; ++i) {
        let curr_node = data[i];
        curr_node.x = width/2;
        curr_node.y = Math.max(dotradius, Math.min(height - dotradius, curr_node.y));
      }
		}                                      
        
    for(let i=0; i<100; i++) force.tick();
                                  
    svg.append("g").classed("post", true).selectAll("circle")
    	.data(data)
    	.enter()      
    		.append("circle") 
    		.attr("cx", d => d.x + dotradius * 5)
    		.attr("cy", d => d.y)
    		.attr("r", dotradius)
    		.style("fill", "red")
    		.style("stroke", "red")
    		.style("stroke-opacity", 0.5)
    		.style("fill-opacity", 0.5)

</script>
</body>

https://d3js.org/d3.v4.min.js