javascript - Passing Object function into data() in d3 -

http://bl.ocks.org/mbostock/1134768

hey guys, i'm new javascript, , learning use d3 render data. i'm trying understand what's going on in code above, in particular, in snippet:

// add rect each date var rect = cause.selectall("rect") .data(object) // weird me.... .enter().append("svg:rect") .attr("x", function(d) { return x(d.x); }) .attr("y", function(d) { return -y(d.y0) - y(d.y); }) .attr("height", function(d) { return y(d.y); }) .attr("width", x.rangeband()); 

what's object constructor doing in .data()? think data() force evaluation of function, in effect object being created? why needed insert rectangle each element of each array in causes?

see this answer.

it being used identity function - bound selection before, remains bound. pattern necessary because have call .data() update selection.

personally dislike obfuscating code this; rather .data(function(d) { return d;}) obvious i'm doing there. ymmv.