svg.selectAll("placeholder-for-rects") // This will select all placeholder elements and hand them off to the next step
.data(dataset) // This will bind data to the selected placeholder elements, and then return references to all elements to which data was just bound. References to these elements are called “update selection”.
.enter() // Select elements that do not yet exist and hand them off to the next step.
.append("rect") // Create a rect element within each element returned by enter(), and then return reference to this newly created element.