The event isn’t actually overridden, but both are triggered — the onclick handler for the SVG and for the bar. To prevent this, use the .stopPropagation() method (see the documentation). The code looks like this:
rect.on("click", function() {
console.log("rect");
d3.event.stopPropagation();
});
Complete example here. Compare with the behaviour with stopping the propagation of the event here.