D3.js linear scale, domain & range explanation

var data =[64, 62, 63, 59, 60, 57];
 
var linear = d3.scale.linear()
.domain([50, 70])
.range([1, 50]);

for(var i=0; i<data.length; i++)
    console.log(linear(data[i])); //==> 35.3,30.4,32.85,23.05,25.5,18.15
    
    Dmin = 50;
    Dmax = 70;
    
    Rmin = 1;
    Rmax = 50;
    
    Ddiff = Dmax - Dmin = 20;
    Rdiff = Rmax - Rmin = 49;
    
    I = Rdiff / Ddiff = 49 / 20 = 2.45
    
    x = (V - Dmin) * I + Rmin; //(64 - 50) * 2.45 + 1 = 35.3