How to smooth two curves at joint point in d3.js v4?

I solved my initial goal to have the last point to be joint by a dashed line, while the rest of the curve is joint with solid line (see the image).

But, by doing that as follows, I lost the smooth of the curve. How would you address this?

In addition, what to do for having a style supersedes a class like it does in HTML5? (the circle has a red stroke as a style attribute but win the class it has a blue one).

    // curve
    self.svg.append("path")
    .datum(function() {return data.slice(0,data.length-1);})
        .attr("stroke", colors(i)) 
        .attr("class", "line ")
        .attr("d", d3.line()
            .curve(d3.curveCatmullRom)
            .x(function(d) { return self.xSales(d.period) + self.xSales.bandwidth()/2;})
            .y(function(d) { return self.ySales(v(d)); })
        );

    self.svg.append("path")
    .datum(function() {return data.slice(data.length-2, data.length);})
        .attr("stroke", colors(i)) 
        .attr("class", "line currentPeriod")
        .attr("d", d3.line()
            .curve(d3.curveCatmullRom)
            .x(function(d) { return self.xSales(d.period) + self.xSales.bandwidth()/2;})
            .y(function(d) { return self.ySales(v(d)); })
        );

Solution

With regards to my comment:

For your first question, you'll need to generate a single path and use dash-offset and a gradient coloring.

I provided a pretty good answer here about how to dash a section of single path but it doesn't provide the colors. So I've updated below for that:

<!DOCTYPE html>
<meta charset="utf-8">
<style>
  body {
    font: 10px sans-serif;
  }
  
  .axis path,
  .axis line {
    fill: none;
    stroke: #000;
    shape-rendering: crispEdges;
  }
  
  .x.axis path {
    display: none;
  }
  
  .line {
    fill: none;
    stroke-width: 1.5px;
  }
</style>

<body>
  <script src="//d3js.org/d3.v3.min.js"></script>
  <script>
    var margin = {
        top: 20,
        right: 20,
        bottom: 30,
        left: 50
      },
      width = 960 - margin.left - margin.right,
      height = 500 - margin.top - margin.bottom;

    var data = d3.range(11).map(function(d, i) {
      return {
        x: i,
        y: Math.random() * 100
      };
    });

    var x = d3.scale.linear()
      .range([0, width])
      .domain([0, 10]);

    var y = d3.scale.linear()
      .range([height, 0])
      .domain([0, 100]);

    var xAxis = d3.svg.axis()
      .scale(x)
      .orient("bottom");

    var yAxis = d3.svg.axis()
      .scale(y)
      .orient("left");

    var line = d3.svg.line()
      .x(function(d) {
        return x(d.x);
      })
      .y(function(d) {
        return y(d.y);
      })
      .interpolate("basis");

    var svg = d3.select("body").append("svg")
      .attr("width", width + margin.left + margin.right)
      .attr("height", height + margin.top + margin.bottom)
      .append("g")
      .attr("transform", "translate(" + margin.left + "," + margin.top + ")");
      
    var lG = svg.append("defs")
      .append("linearGradient")
      .attr("id", "lineColor")
      .attr("x1", "0%")
      .attr("x2", "100%")
      .attr("y1", "0%")
      .attr("y2", "0%");
    
    lG.append("stop")
      .attr("offset", "0")
      .attr("stop-color", "steelblue");
      
    svg.append("g")
      .attr("class", "x axis")
      .attr("transform", "translate(0," + height + ")")
      .call(xAxis);

    svg.append("g")
      .attr("class", "y axis")
      .call(yAxis)

    var p = svg.append("path")
      .datum(data)
      .attr("class", "line")
      .attr("stroke", "url(#lineColor)")
      .attr("d", line);

    // draw dashed from 2.7 to 7 in the X domain
    var dashBetweenX = [2.5, 7]
    path = p.node(),
      totalLen = path.getTotalLength();

    // find the corresponding line lengths
    var dashBetweenL = dashBetweenX.map(function(d, i) {

      var beginning = 0,
        end = totalLen,
        target = null,
        d = x(d);

      // find the line lengths the correspond to our X values
      // stolen from @duopixel from http://bl.ocks.org/duopixel/3824661
      while (true) {
        target = Math.floor((beginning + end) / 2);
        pos = path.getPointAtLength(target);
        if ((target === end || target === beginning) && pos.x !== d) {
          break;
        }
        if (pos.x > d) end = target;
        else if (pos.x < d) beginning = target;
        else break; //position found
      }

      return target;
    })

    var sd =  dashBetweenL[0],
        dp = dashBetweenL[0],
        count = 0;
    while (dp < dashBetweenL[1]) {
      count++;
      dp += 4;
      sd += ", 4";
    }

    if (count % 2 == 0) {
      sd += ", 4, " + (totalLen - dashBetweenL[1]);
    } else {
      sd += ", " + (totalLen - dashBetweenL[1]);
    }
    p.attr("stroke-dasharray", sd);
    
    lG.append("stop")
      .attr("offset", dashBetweenX[0] / x.domain()[1])
      .attr("stop-color", "steelblue");
    
    lG.append("stop")
      .attr("offset", dashBetweenX[0] / x.domain()[1] + 0.001)
      .attr("stop-color", "red");
      
    lG.append("stop")
      .attr("offset", dashBetweenX[1] / x.domain()[1])
      .attr("stop-color", "red");
      
    lG.append("stop")
      .attr("offset", dashBetweenX[1] / x.domain()[1] + 0.001)
      .attr("stop-color", "steelblue");
      
    lG.append("stop")
      .attr("offset", '1')
      .attr("stop-color", "steelblue");
    
  </script>
</body>

</html>