Interactivity chapter reorganization

[jtr13]

7.2 change graphic to a bunch of circles on the svg

7.2.1 d3.select("svg").selectAll("circle").on("click")
(explain: message is added if someone clicks on any circle point: we don't need any information about which circle or its attributes, data, etc.)

7.2.2 change an attribute
d3.select("svg").selectAll("circle").on("click") ...
.attr("fill", "blue");
clicking on a circle changes its attribute (you need to know which circle)

7.2.3 get information about the element that received an event
d3.select("svg").selectAll("circle")
use either cx or r -- should be something that is different for the different circles

7.2.4.
d3.select("svg").selectAll("circle").data([ single array of different values than current ones for radii]).on("click", function(event, d) {
d3.select(event.currentTarget).attr("r", d); ...