What Are Line Plots and Line Graphs?

Purpose

• The purpose of a line plot is to summarize a relatively small number (under about 50, there is no exact upper limit, but over about 50 the plot gets very hard to read) of observations of a numeric variable, such as weight. A line plot is like a visual frequency table.

  The purpose of a line graph is to track the course of a continuous variable over time.

Examples

• A line plot would be a good way to display the weights of all the students in a class, unless the class were very large. Line plots could also be used to display the incomes of all the people in a small apartment building.

  A line graph could be used to show the closing price of a stock on consecutive days, or the low temperature on consecutive days or the number of cars sold in the United States in consecutive years.

Limitations

• Line plots are limited in the total sample size they can deal with. They can also deal only with one variable and cannot display relationships among variables. In addition, if the values of the variable have large gaps, a line plot can be cumbersome.

  Line graphs show the relationship among two variables, one of which is time. They cannot account for other variables. Simple line graphs may be very "choppy" if the data are not smooth -- that is, if values go up and down rapidly.

Alternatives

• If you have more data than will fit in a line plot, alternatives include strip plots, histograms, density plots and box plots. If you want to look at the relationship of a quantitative variable and a categorical one, parallel box plots can be useful. To look at the relationship between two continuous variables, a scatter plot is standard.

  Line graphs can sometimes be improved by adding a smooth line; there are many methods of smoothing, including moving averages and loess, which is a complex method of smoothing that works like a weighted moving average.

Appearance

• A line plot looks like a number line with a bunch of Xs over the line at various points. The number line is marked with values for the continuous variable and each X represents a subject (for example, a person).

  A line graph looks like a graph with the horizontal axis marked for time and the vertical axis marked with levels of the other variable.