d3拖动事件导致TypeError

Question

I am trying to enable dragging in my chart, but I only got dragstart and dragend events.

something strange is that if I use the .origin method as in the examples, it throws an exception (Uncaught TypeError: Cannot read property 'x' of undefined ):

this._drag = d3.behavior.drag()
//.origin(function(d) { return d;}) //if uncommented, throws error!
.on("drag", function(d){console.log("drag!", d); self.on_drag(this);})
.on("dragstart", function(){self.on_drag_start(this);})
.on("dragend", function(){self.on_drag_end(this);})
;

Answer 1

the problem with origin was caused by the fact that I was trying to drag a svg element that was not part of the chart itself, but a decorator item (to emulate a icon) that I added separately. thus, the element has not a datum (d is undefined) but if you return a custom object with x and y attributes, you can use the drag behaviour normally.

thanks to the people on the #d3.js irc channel for helping me to figure this out!