Learn to Create a Radar Chart in R With Data From Our World in Data (2018) © 2021 SAGE Publications, Ltd. All Rights Reserved. This PDF has been generated from SAGE Research Methods Datasets. SAGE SAGE Research Methods: Data 2021 SAGE Publications, Ltd. All Rights Reserved. Visualization Learn to Create a Radar Chart in R With Data From Our World in Data (2018) Student Guide Introduction This guide explores creating a radar chart that compares energy consumption by type across a chosen set of countries. Radar charts are used to visualize and compare the distribution of quantitative values across a series of categories. The radar chart uses primarily marker distance from the axis to encode value, even though the most striking visual components of the output are irregular shapes with variable surface area. The visualization in this tutorial (Figure 1) uses data from Our World in Data. The individual point locations represent values across a series of different energy source categories, whereas the subsequent filled area created between these points does not directly encode value as such . The charts show data for three countries. The data from the chart, in the decreasing order of the source quantity, are as follows. Estonia: 77 terawatt-hours • Coal • Oil • Natural gas Page 2 of 18 Learn to Create a Radar Chart in R With Data From Our World in Data (2018) SAGE SAGE Research Methods: Data 2021 SAGE Publications, Ltd. All Rights Reserved. Visualization Latvia: 37 terawatt-hours • Oil • Natural gas • Hydropower Lithuania: 64 terawatt-hours • Oil • Natural gas • Coal Text under the charts reads, “Source: Our World in Data, 2018.” Figure 1. A Radar Chart What Is a Radar Chart? The radar chart maps three or more different quantitative variables across a series of categories at equal intervals along a circle. The quantity in each category is marked by a marker plotted a certain distance away from the central axis, which is then connected sequentially to the other category value markers to create a Page 3 of 18 Learn to Create a Radar Chart in R With Data From Our World in Data (2018) SAGE SAGE Research Methods: Data 2021 SAGE Publications, Ltd. All Rights Reserved. Visualization unique shape, which can be either filled or unfilled. The angles between the points only represent qualitative distinctions, and the overall shape and surface area depend largely on the arrangement of the categories—therefore not actually encoding total value across categories, though it may at first glance seem to do so. Only the distance of each marker from its origin encodes value, and comparisons between categories are similarly based solely on this distance. Radar charts can be compared side-by-side provided that the axes’ maximum and minimum values are identical across the board. Values should preferably be percentages, but quantitative values will also suffice provided that there are no significantly outlying values in the dataset. Negative values and axes starting from anything other than zero are generally not recommended, as they render the charts considerably more challenging for the reader to comprehend. Sometimes, the radar chart type is also called a spider or web chart due to its resemblance to a spider’s web. The term radar chart is often used interchangeably and confused with the polar chart, which for our purposes is an altogether different chart type, showing quantitative variables using a line drawn on a polar coordinate system. One variable (typically time) is mapped to the angle (theta, Θ), and the other variable to the radius (r) (Figure 2). The polar chart shows weeks 1 through 52, in increments of 4, along the edge and from 0 to 450 in increments of 75 from the center to the edge. All data are approximate. For all years, the number of deaths fluctuated at 100 between weeks 16 and 48. In 2015, the number of deaths increased to 225 in week 4 and then decreased. In 2016, the number of deaths was at 150 between weeks 1 and 12 and then decreased. In 2017, the number of deaths was at 240 in week 2 and then decreased. In 2019, the number of deaths was at 450 in week 1 and then decreased. Text under the chart reads, “Source: NCHS 2018.” Page 4 of 18 Learn to Create a Radar Chart in R With Data From Our World in Data (2018) SAGE SAGE Research Methods: Data 2021 SAGE Publications, Ltd. All Rights Reserved. Visualization The radar chart lists oil, other renewables, wind, solar, hydropower, nuclear, and coal. The chart shows that Latvia uses the following energy sources in the decreasing order of quantity: Oil, natural gas, and coal. Text under the chart reads, “Our World in Data, 2018.” Figure 2. Polar Chart (left) and Radar Chart (right) Why Use a Radar Chart The radar chart is a relatively rarely used chart type, which has the potential to stand out when competing for attention in a crowded media environment, such as in a poster session. Its use should, however, be carefully weighted against the fact that other chart types greatly outperform the radar chart in most use cases, particularly in terms of readers’ accurate assessment of values. The radar chart is best reserved for use as a very general overview of value distribution across a series of categories. It should also be considered that the reader may not be familiar with the radar chart type and may not intuitively understand how to read it. Page 5 of 18 Learn to Create a Radar Chart in R With Data From Our World in Data (2018) SAGE SAGE Research Methods: Data 2021 SAGE Publications, Ltd. All Rights Reserved. Visualization Considerations and Cautions While radar charts may be visually striking, they can be hard to interpret as the reader is tempted to evaluate value based on shapes and surface areas—which in this case are not accurately indicative of underlying value and are inherently more difficult to assess than length, for example. The chart type is furthermore prone to misleading the reader, as many plotting methods allow the center of the plot to be set to other values than zero in order to emphasize variation—such as truncating the axis or moving out the zero value to the inner-most gridline. It is recommended as much as possible to set the zero value in the dead center and clearly mark whenever this is not the case. It is furthermore recommended to label the grid and/or the value markers themselves whenever possible, though sadly most methods of generating radar charts do not do this by default and require some excess amount of coding or editing outside of the programming environment. It is possible, though not necessarily advisable, to plot several sets on one background grid. In such cases, it is best to use a qualitative color scheme to distinguish each set and semi-transparent fill and stroke colors to keep legibility at a maximum. An alternative way to compare different sets is to plot several radar charts side-by-side in a small multiples layout, in which case, the same fill and stroke colors can also be used for all. Variations and Alternatives As mentioned earlier, the terms radar chart and polar chart are often used interchangeably and resemble each other visually. Here the term polar chart is reserved for a chart which shows quantitative variables using a line drawn on a polar coordinate system so that one variable (typically time) is mapped to the Page 6 of 18 Learn to Create a Radar Chart in R With Data From Our World in Data (2018) SAGE SAGE Research Methods: Data 2021 SAGE Publications, Ltd. All Rights Reserved. Visualization angle (theta, Θ) and the other to the radius (r). Theta is usually defined so that a full revolution in the coordinate system corresponds to a time period of fixed length (such as a year or 24 hours). A data variable that remains the same over the plotted time period will thus draw a circle, while a continuously increasing variable draws a spiral (Figure 3). The polar chart shows weeks 1 through 52, in increments of 4, along the edge and from 0 to 450 in increments of 75 from the center to the edge. All data are approximate. For all years, the number of deaths fluctuated at 100 between weeks 16 and 48. In 2015, the number of deaths increased to 225 in week 4 and then decreased. In 2016, the number of deaths was at 150 between weeks 1 and 12 and then decreased. In 2017, the number of deaths was at 240 in week 2 and then decreased. In 2019, the number of deaths was at 450 in week 1 and then decreased. Text under the chart reads, “Source: NCHS 2018.” Figure 3. Polar Chart Page 7 of 18 Learn to Create a Radar Chart in R With Data From Our World in Data (2018) SAGE SAGE Research Methods: Data 2021 SAGE Publications, Ltd. All Rights Reserved. Visualization If the original data is in polar coordinates, it should also be drawn on a polar coordinate system. Such applications of polar charts include polar radiation plots for antennae and wind speed and direction charts (wind roses). Better alternatives for visualizing the types of datasets shown in a radar chart Page 8 of 18 Learn to Create a Radar Chart in R With Data From Our World in Data (2018) SAGE SAGE Research Methods: Data 2021 SAGE Publications, Ltd. All Rights Reserved. Visualization might be, for instance, the pictorial unit chart, waffle chart, or stacked bar chart. A pictorial unit chart, also known as a pictogram chart or isotype chart, consists of a variety of shapes or glyphs representing absolute value—for example, where each house symbol represents 100 households.