protovis part 2: lines and labels

Next, we need to add a data array to describe the distribution of the tick lines. in this case we want lines at 0, .5, 1 and 1.5. The tick lines need to be distributed from the bottom so type .bottom(function (d) d * 60 + 15) to start the lines 15 pixels from the bottom. Notice that this is the same function that we used to describe the height of the bars.