Spaghetti Plot

The purpose of the spaghetti plot is to present the trajectory or trend of data. This display can take the form of a graph or a figure, depending on the data. Numerous long lines that curve and intersect give the appearance of noodles, hence the name, spaghetti plot. Typically, whether data are to be presented over time or over space (such as in a map) will determine whether the spaghetti plot takes the form of a graph or a figure. A graph displays individual responses over time for psychological or physiological (interval or ratio) variables. In a figure, the physical path of the variable of interest is important to display to provide an indication of how the variable’s path changes through space. This entry explains the usage of the spaghetti plot in the graph and figure forms and the rationale for its use, provides examples, and presents the advantages and disadvantages of this type of visual data presentation.

In graph form, the spaghetti plot typically reflects longitudinal, repeated measures of individual raw scores (see Figure 1). This is plotted on a chart where the dependent variable, or variable of interest, is on the y-axis, while time is on the x-axis. The dots for a participant’s data points are connected to form a line. Depending on the research design and purpose of the study, another variable of interest, other than time, can be displayed on the x-axis. The spaghetti plot is distinct from a line graph as the points on a line graph present summarized data. It is also different from a scatterplot, which is frequently used to present discrete data points that reflect a pattern in the overall data but does not permit comparisons of individuals’ responses over time.

Examples of graphical spaghetti plots can be found in medicine, psychology, education and management, whereby the outcome variable is tracked over days, months, or years for numerous individuals. The outcome variables are, for instance, psychological or physiological variables such as weight, physical symptoms, neurological outcomes, biological outcomes, cognitive functioning, and psychological symptoms. The purpose is to determine whether the outcome variable trajectory presents itself in a similar manner for the participants over time. The trend of an individual’s data is of interest, as well as that of the entire sample or subsample. Although it is typical to employ the spaghetti plot to illustrate the measured variable more than twice, it is also employed when there are only two points of data for each participant.

Spaghetti plots presented in a figure are designed to present individual data, as well, but not over time. In a figure, the objective is to illustrate the actual environment in relation to the individual data. Examples of data presented in a figure can be found in management, human factors and ergonomics, and engineering, where movement or flow is illustrated via paths on a map. For instance, movement in an office floor plan may be important to track (see Figure 2). In meteorology, each line may reflect different weather pressures or different actions of weather systems illustrating distinct paths for these weather phenomena over a map.

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