Solved – Is it wrong to use line plots for discrete data

data visualization

I've often seen discrete datasets plotted as line plots, but it occurs to me that the line infers a value at a point between the measurement intervals which is meaningless for discrete datasets. Is it therefore the case the using line plots for discrete data is wrong?

As an example, take two time-series datasets, one continuous (my weight, measured daily in the morning) and one discrete (the number of donuts I eat per day). It makes sense for the first dataset to be a line plot, as it is reasonable to infer that my weight in any given afternoon will be related to my weight the preceding and following mornings. However, if the number of donuts is represented as a line graph the lines between the dots no meaning can be inferred from that line.

EDIT

Here's another example: the Federal Hourly Minimum Wage Since Its Inception plot at http://mste.illinois.edu/courses/ci330ms/youtsey/lineinfo.html

Unless I'm mistaken, the minimum wage changes are discrete, and hence it's not possible to look up some arbitrarily selected time and establish the minimum wage at the point using the line interconnecting the dots.

Best Answer

Connected line plots have proven too useful to limit to a single interpretation. A few prominent uses:

  • Interpolated values. The case you mention where both variables are continuous and every interpolated point along the line as a meaningful interpretation.
  • Rate of change. Even when the in-between values aren't meaningful, the slope of each line segment is a good representation of the rate of change. Note that for this interpretation, the X and Y values must be spaced appropriately, which is not the case in the wage plot you cite.
  • Profile Comparison. When comparing small multiples or overlaid measures, lines can be useful even for categorical factors. In this case, the lines serve to connect groups of responses for limited pattern recognition. Here's an example from peltiertech.com with the factor on the Y (instead of the X) axis for label readability:

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