Lesson 4: Aggregating Data Elements

In the previous lesson, you learned that elements are "lightweight" objects that visually represent underlying concepts. In the previous example, elements appeared to have a one-to-one correspondence with the underlying concept. For example, there was one "Birmingham" element that had population 883,946. However, an extremely powerful feature in Visiage is the ability to create new elements by grouping other elements. This feature is called aggregation.

Aggregating elements is very simple. The first step is to select those elements you wish to aggregate. This is accomplished by choosing a paint color from the Marking palette, and then clicking each element in the desired group (see Lesson 2 for a review of selection and the Marking palette).

Next, shift-drag (or drag) the selected elements to the Compose button in the Tools frame. (If you drag the elements, the individual elements will be removed and a new element created. If you shift-drag the elements, the individual elements will remain unchanged and a new element created.) Drop the elements over the Compose button. A dialog box will appear to let you choose how to aggregate the elements and what to name the new element.

Aggregated elements are themselves lightweight objects that can be moved from frame to frame, copied, removed, or even used themselves in other aggregations.

• If necessary, drill down in the "cities" database so that all of the cities in Alabama are rendered as elements (as shown in the previous lesson).
  

• Create a new Outliner frame.
• Select the four main cities in Alabama by choosing a selection color from the Marking frame and clicking on Birmingham, Huntsville, Mobile, and Montgomery.
• Shift-drag these cities to the new Outliner frame by putting your mouse pointer over one of the selected elements and dragging to the new frame.
  

• Shift-drag the cities from the new Outliner frame to the Compose icon in the Tools Frame.
• A dialog box will appear with a field to enter a name for the aggregate.
  

• Name the new aggregate "four_cities" by entering this in the "Agg Name:" text input field.
• Click OK.. Visage will create a new element that represents the four cities.
• Display the "members by description" relation of the new aggregate element.

One way to imagine using this feature might be to aggregate four cities together in order to see their population as a group. However, this functionality is not yet supported in Visage, as it is not clear how to represent attributes when merged. For example, you might merge the four cities' population as a sum. You might also choose to average the population, or possibly choose the minimum or maximum values. Since Visage doesn't know which type of operation you are interested in, it does not attempt to create attribute values for the aggregate that correspond to the member elements.

At this point, you should consider aggregation to be a shorthand method for grouping elements. When you begin exploring real databases (e.g., RFTAGs) other benefits of aggregation will become apparent.

Summary

In this chapter, you learned how to aggregate elements. This process creates another element that can be manipulated in the same way as any other Visage element. It can be dragged, moved, copied, or aggregated with other elements to form a new aggregate element.

Up to this point, you have explored Visage primarily in the context of the Outliner frame. In the next chapter, you will learn about the Simple Plot frame.