Multiple line segments in the PCP may be selected simultaneously by click-dragging a box around the bundle. Upon release of the mouse button, the observations of which the boxed line segments are a part are highlighted. This technique is called "brushing." This can be a very effective method of focusing attention on the relative signatures of a group of observations that share similar values of one attribute. In the example at left, the traces for all observations which took place on a particular day (note that the "sequential_date" radio button is selected) are highlighted. By following these highlighted traces across the PCP, a researcher can make a rapid qualitative assessment of the character of the variables on that day (e.g. "this was a wet day across the region," or "the southern locations experienced a significant barometric pressure increase on this day.")
In the larger Apoala visualization system, this operation is linked with other representation forms; that is, performing this operation on the PCP will alter maps, scatter plots, and other representation forms in a similar way.
