Abstract
This chapter deals with the most common method of presenting time in a plot of data – using it as a coordinate to locate elements of the graphic. The time plot, with time on the horizontal dimension and a continuous value plotted as a moving line on the vertical axis, epitomizes the power of this principle. Most books on time series analysis will lead off with time series displays, inviting the reader to observe them and then using the plots to motivate the following analysis. In this chapter we will start at a more basic level – the 1-D chart with time as a single dimension. Variants and decorations for this very simple chart will be discussed; aesthetics that map other variables play a key role, as do positional modifiers that stack elements and the use of additional elements to indicate patterns. Sometimes a second dimension will be used, straying into the territory of the next chapter, but the thrust of this chapter is to explore how time can be mapped to a single coordinate so as to show patterns and enable action.
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Notes
- 1.
Mallet was a mathematics graduate of Trinity College Dublin, as was I. However, he graduated 150 years before I did and managed to do so when he was only 20 years old. He coined the words “seismology” and “epicenter” and was a major researcher in this area.
- 2.
Another minor change: The filled circles have been changed to open circles to help mitigate overplotting.
- 3.
The term identifier is used for a categorical variable that has a different value for each row in the data. It cannot be used for any analytic purpose, but it can be used to identify items in the data. An example would be people’s names in a census or survey, or the text of Twitter comments in this example.
- 4.
This is a simple and effective general technique for modifying plots that have significant overplotting. The degree of transparency is a tunable parameter. If the elements are relatively opaque, then single items will be clearer and so outliers and unusual values will be more distinct, but differences in dense areas will be hard to perceive. Conversely, high transparency makes single values harder to observe but aids comparison of densely overplotted regions.
- 5.
It might form an interesting problem for statistical students …
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Wills, G. (2010). Time as a Coordinate. In: Visualizing Time. Statistics and Computing. Springer, New York, NY. https://doi.org/10.1007/978-0-387-77907-2_5
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DOI: https://doi.org/10.1007/978-0-387-77907-2_5
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