Data visualization consultant Stephen Few has just posted an excellent article [perceptualedge.com] about a recent time-series visualization called the Horizon Graph. Originally developed by data visualization software firm Panopticon, Horizon Graphs can display about 50 sets of time-series values on a single screen. This particular visualization technique was the focus of a detailed evaluation study described by Jeffrey Heer, graduate researcher Nicholas Kong, and Maneesh Agrawala. The results are described in the scientific paper titled "Sizing the Horizon: The Effects of Chart Size and Layering on the Graphical Perception of Time Series Visualizations" (PDF) [berkeley.edu].
In the paper, 2 different experiments are described. The goal of the first experiment was to determine the impact of the band number and horizon graph variant ("mirrored", flipping the negative values around zero, versus "offset") on value comparisons between horizon graphs. The goal of the second experiment was to compare normal line charts to horizon graphs and investigate the effect of chart height on both.
Want to know the dry results? No significant difference was found in either estimation time or accuracy between the different chart types. However, both estimation time and error increased as more bands were displayed. In the 2nd experiment, the estimation error increased as chart size decreased; layering increased estimation time, and mirroring did not; and lastly, the estimation time decreases with chart height.
More interestingly, from the results, following 3 design guidelines were proposed:
- Mirroring Does Not Hamper Graphical Perception. Mirroring a chart (that is flipping the negative values around zero) neither slowed estimation time nor hurt estimation accuracy, but cuts the size of the chart in half.
- Layered Bands Are Beneficial As Chart Size Decreases. Dividing a chart into layered bands increased the estimation time and increased the estimation error at constant chart heights. Therefore, the use of 4 or more bands is discouraged, as this resulted in increased time and error, while subjects complained that interpreting 4-band charts was difficult and tiring
- Optimal Chart Sizing. For both normal line charts and 1-band mirror charts, they found a chart height of 24 pixels to be optimal.
UPDATE: Jeffrey Heer pointed out some details that might be misinterpreted: "We do recommend a height of 24 pixels, but this is assuming a 14.1 inch 1024 x 768 display. Given the wide variety of display types and resolution, the size in millimeters (6.8mm) is likely a more informative measure for others to adopt." Thanks for the great research, Jeffrey!