061-Data Visualisation数据可视化(四)
The Structure of a Graphic
Writing in 1967, Bertin focused on flat, static information graphics. Fifty years of transformational technology have done little to change this paradigm—our work remains bound to the two-dimensional planes of paper and screens. In this light, there are two critical ingredients to any graphic: the visual mark or implantation itself (for example, a dot on a scatter plot graph) and its position on the plane (where the dot is placed relative to the x-axis and y-axis that bounds the graphic).
Here, the invariant (topic) is “estimated gun sales in America since 2000”—the overarching content of the graphic. Its components are a variation in sales and a variation in time. Its elements are estimated gun sales at each given month. Note the relationship between the invariant and the title of the graphic, between the components and the axes and planar space of the graphic, and the elements and the visual implantations (marks).
Levels of Organization
Components themselves are organized on one of three levels:
- Quantitative: Component consists of elements with constant numerical ratios between one another, like a sequence of minutes.
- Ordered: Component consists of elements with a natural sequence, like cold-warm-hot or white-gray-black.
- Qualitative: Component consists of associated and differentiated groups, yet has no inherent order, like a list of the fifty states in America. Useful: as qualitative components have no inherent order, they can be arbitrarily re-ordered to reveal patterns in the data.
Preattentive Processing
For many years vision researchers have been investigating how the human visual system analyses images. An important initial result was the discovery of a limited set of visual properties that are detected very rapidly and accurately by the low-level visual system. These properties were initially called preattentive, since their detection seemed to precede focused attention. We now know that attention plays a critical role in what we see, even at this early stage of vision. The term preattentive continues to be used, however, since it conveys an intuitive notion of the speed and ease with which these properties are identified.
Typically, tasks that can be performed on large multi-element displays in less than 200 to 250 milliseconds (msec) are considered preattentive. Eye movements take at least 200 msec to initiate, and random locations of the elements in the display ensure that attention cannot be prefocused on any particular location, yet viewers report that these tasks can be completed with very little effort. This suggests that certain information in the display is processed in parallel by the low-level visual system.
A simple example of a preattentive task is the detection of a red circle in a group of blue circles (Fig. 1). The target object has a visual property "red" that the blue distractor objects do not (all non-target objects are considered distractors). A viewer can tell at a glance whether the target is present or absent.
Similarity
The most intuitive gestalt principle is that graphical elements with shared visual properties will be considered in the same group. Here we see the use of color similarity to indicate two classes of elements: the red ones and the gray ones. This could have also utilized shared symbols (for instance leveraging d3.svg.symbol or the like) to show shared category; or shared stroke color or width; icons and so on.
Proximity
A graphical element being close to another graphical element is a strong indication of similarity. The circles on the right have been split into two groups by simply making the 10 circles on the left closer to each other than the 30 circles on the right.
Enclosure
The use of enclosure--surrounding a group of related elements with a visual element--is not a common technique in data visualization. This is remarkable given how powerful enclosure is. Here we see enclosure alongside similarity and proximity and yet providing the strongest visual signal of the three.
