To make animation in d3js you usually have an interplay between multiple attributes of objects. This simple solution should simplify the process of animation. You just connect a value
var bindIncrease = new d3binding();
bindIncrease.ease = "elastic"
bindIncrease.duration = 2000
bindIncrease.connect(line, "x2", function(v) {return 100 + v;});
bindIncrease.connect(dot_center, "cx", function(v) {return 100 + v;});
bindIncrease.connect(dot_extra, "cx", function(v) {return 3 * v;});
bindIncrease.connect(hline, "x2", function(v) {return 3 * v;});