protovis part 2: lines and labels
Positioning the lines
We want the tick lines to be positioned identically to the baseline, so copy the left and right settings from the baseline and paste them on the end of the tick lines chain. Now type .strokeStyle ("lightgray") to change the color.
Notice that the baseline is covered up by the tick lines. In Protovis, whatever is last in your code is on top of what preceded it. In this case, the bars are in the back, the baseline rule is on top of the bars and the tick lines are on top of the baseline rule and the bars.
To move the tick lines behind the bars, select the tick lines code, cut it and paste it above the bars.