href="#x_9_i110">Figure 1.15, we see Shine Pulikathara's visualization that won the 2015 Tableau Iron Viz competition. Notice his simple use of color.
Figure 1.15 Winning visualization by Shine Pulikathara during the 2015 Tableau Iron Viz competition.
Source: Used with permission from Shine Pulikathara.
Color should be used purposefully. For example, color can be used to draw the attention of the reader, highlight a portion of data, or distinguish between different categories.
Use of Color
Color should be used in data visualization in three primary ways: sequential, diverging, and categorical.
In addition, there is often the need to highlight data or alert the reader of something important. Figure 1.16 offers an example of each of these color schemes.
Figure 1.16 Use of color in data visualization.
Sequential color is the use of a single color from light to dark. An example is encoding the total amount of sales by state in blue, where the darker blue shows higher sales and a lighter blue shows lower sales. Figure 1.17 shows the unemployment rate by state using a sequential color scheme.
Figure 1.17 Unemployment rate by state using a sequential color scheme.
Diverging color is used to show a range diverging from a midpoint. This color can be used in the same manner as the sequential color scheme but can encode two different ranges of a measure (positive and negative) or a range of a measure between two categories. An example is the degree to which electorates may vote Democratic or Republican in each state, as shown in Figure 1.18.
Figure 1.18 Degree of Democratic (blue) versus Republican (red) voter sentiment in each state.
Diverging color can also be used to show the weather, with blue showing the cooler temperatures and red showing the hotter temperatures. The midpoint can be the average, the target, or zero in cases where there are positive and negative numbers. Figure 1.19 shows an example with profit by state, where profit (positive number) is shown in blue and loss (negative number) is shown in orange.
Figure 1.19 Profit by state using a diverging color scheme.
Categorical color uses different color hues to distinguish between different categories. For example, we can establish categories involving apparel (e.g., shoes, socks, shirts, hats, and coats) or vehicle types (e.g., cars, minivans, sport utility vehicles, and motorcycles). Figure 1.20 shows quantity of office supplies in three categories.
Figure 1.20 Quantity of office supplies in three categories using a categorical color scheme.
Highlight color is used when there is something that needs to stand out to the reader, but not alert or alarm them. Highlights can be used in a number of ways, as in highlighting a certain data point, text in a table, a certain line on a line chart, or a specific bar in a bar chart. Figure 1.21 shows a slopegraph with a single state highlighted in blue.
Figure 1.21 Slopegraph showing sales by state, 2014–2015, using a single color to highlight the state of Washington.
Alerting color is used when there is a need to draw attention to something for the reader. In this case, it's often best to use bright, alarming colors, which will quickly draw the reader's attention, as in Figure 1.22.
Figure 1.22 Red and orange indicators to alert the reader that something on the dashboard needs attention.
It is also possible to have a categorical-sequential color scheme. In this case, each category has a distinct hue that is darker or lighter depending on the measurement it is representing. Figure 1.23 shows an example of a four-region map using categorical colors (i.e., gray, blue, yellow, and brown) but at the same time encoding a measure in those regions using sequential color; let's assume that sales are higher in states with darker shading.
Figure 1.23 Sales by region using four categorical colors and the total sales shown with sequential color.
Color Vision Deficiency (Color Blindness)
Based on research (Birch 1993), approximately 8 percent of males have color vision deficiency (CVD) compared to only 0.4 percent of females. This deficiency is caused by a lack of one of three types of cones within the eye needed to see all color. The deficiency commonly is referred to as “color blindness”, but that term isn't entirely accurate. People suffering from CVD can in fact see color, but they cannot distinguish colors in the same way as the rest of the population. The more accurate term is “color vision deficiency.” Depending on which cone is lacking, it can be very difficult for people with CVD to distinguish between certain colors because of the way they see the color spectrum.
There are three types of CVD:
1. Protanopia is the lack of long-wave cones (red weak).
2. Deuteranopia is the lack of medium-wave cones (green weak).
3. Tritanopia is the lack of short-wave cones (blue). (This is very rare, affecting less than 0.5 percent of the population.)
CVD is mostly hereditary, and, as you can see from the numbers, it primarily afflicts men. Eight percent of men may seem like a small number, but consider that in a group of nine men, there is more than a 50 percent chance that one of them has CVD. In a group of 25 men, there is an 88 percent chance that one of them has CVD. The rates also increase among Caucasian men, reaching as high as 11 percent. In larger companies or when a data visualization is presented
