Q:

How to manually trigger the drag event of d3

All:
Suppose there are two layers( the top one is SVG:PATH, the bottom layer is a SVG:RECT, the top layer covers the bottom layer), I want to apply D3 drag to the RECT layer and mouseover to PATH layer, could anyone show me how to do that?
THE CODE BELOW CAN ONLY WORK WITH THE path LAYER:
        var svg = d3.select("svg");
        svg.style("width", "400px")
            .style("height", "400px")
            .style("border", "1px solid grey");

        var r = svg.select("rect")
            .attr("width", "300px")
            .attr("height", "300px")
            .attr("x", "50px")
            .attr("y", "50px")
            .style("fill", "whitesmoke");

        var p = svg.select("path")
            .attr("d", function(){
                return "M0 0 L380 0 L300 300L0 380Z";
            })
            .style("fill", function(){
                return "rgba(10,10,10,0.2)";
            })
            .on("mousedown", function(){

            });
var drag = d3.behavior.drag();

var dragstart = function(){
    alert("drag start");
};

drag.on("dragstart", );

r.call(drag);

Thanks

A:

I changed your drag. Also changed it to a function :
function drag(){
  console.log('dragging');
    return d3.behavior.drag()
             .origin(function() {
                var g = this;
                return {x: d3.transform(g.getAttribute("transform")).translate[0],
                        y: d3.transform(g.getAttribute("transform")).translate[1]};
            })
            .on("drag", function(d) {

                g = this;
                translate = d3.transform(g.getAttribute("transform")).translate;

                x = d3.event.dx + translate[0],
                y = d3.event.dy + translate[1];

                d3.select(g).attr("transform", "translate(" + x + "," + y + ")");
                d3.event.sourceEvent.stopPropagation();             
            });
}

For some reason this doesnt work on 'codepen' so i put it on JSFiddle and it works fine :)) The problem you had was you had no logic in your Drag functions. So nothing was happening.
Updated fiddle : http://jsfiddle.net/qqb6357j/1/
Here I have just called drag on both the path and rectangle at the bottom of the JS: http://jsfiddle.net/qqb6357j/2/
Just change the selection and call drag on it to give it drag capability :) 
If you want to go one step further and stop all interaction with the 'path' give it no interactivity (you have to give it a class so css can select it):
#path{
    pointer-events:none;
}

Updated fiddle : http://jsfiddle.net/qqb6357j/3/
Now, what you asked for. You said you wanted when mouseover you want to highlight a number of things but be able to drag the layers below it. You can't just turn pointer-event to none as you still want to have 'hover' ability so i created a timeout. This is so when you hover over it, pointer-event=none; for 1 second and after 1 second : pointer-events= all; 
Here is the code :
.on('mouseover', function(){
    p.classed('path', true);
    setTimeout(function() {
        p.classed('path', false);
    }, 1000) //timeout function
    //r.call(drag);
})

Here is the class that gets put on for 1 second : 
.path{
    pointer-events:none;
    opacity: 0.2;
}

Final working fiddle : http://jsfiddle.net/qqb6357j/6/

