A line graph is commonly used to display change over time as a series of data points connected by straight line segments on two axes. The line graph therefore helps to determine the relationship between two sets of values, with one data set always being dependent on the other set.
Line graphs are drawn so that the independent data are on the horizontal a-axis (e.g. time) and the dependent data are on the vertical y-axis. Line graphs are used to track changes over short and long periods of time. There is some debate about the degree of measurement between time points. Some say the data must be measured nearly continually in order for the lines to be accurate representations. Others feel a monthly measurement is sufficient, even though the line implies data at points where no measurement was taken.
Line graphs are useful in that they show data variables and trends very clearly and can help to make predictions about the results of data not yet recorded. They can also be used to display several dependent variables against one independent variable. When comparing data sets, line graphs are only useful if the axes follow the same scales. Some experts recommend no more than 4 lines on a single graph; any more than that and it becomes difficult to interpret.
"Line graphs and scatterplots are similar in that they record individual data values as marks on a graph. The difference between these two formats is how the line is created. In line graphs, the line is created by connecting each individual data point to show local changes, in this way, the local change from point to point can be seen. This is done when it is important to be able to see the local change between pair of points. An overall trend can still be seen, but this trend is joined by the local trend between individual or small groups of points, whereas the line of the scatterplot does not connect individual points but instead shows the trend followed by the data."
Line graphs that connect only two points in time are called slope graphs. These can be handy when you don’t have data for each time period, but want to compare, for example, the start of the program in 1986 to the current state of the program in 2014. Use these if you don’t have other data to show program outcomes.
The difference in median household income between California and the rest of the United States
This example from the California Budget Project is an example of a line chart showing change in median household income in California between 1989 and 2010. The colour difference outlines the difference between California and the United States
Red River discharge rate per month in 1993
Source: Wallace, (2005). www.ncsu.edu/labwrite/res/gh/gh-linegraph.html#multipleline
Advice for CHOOSING this option (tips and traps)
Line graphs are usually used to represent changes over time.
Advice for USING this option (tips and traps)
As best as possible, label the line with its name rather than using a legend.
Highlight important points in the line with the exact value, such as the highest and lowest points or those points where actual data collection occured.
Line Graphs and Scatter Plots: This website from North Carolina State University has guided examples for using line graphs.
Other ways to visualise changes over time
Comparing change between two points in time with a line.
Split Axis Bar graph
Regraphing comparison between two points in time by simply graphing the change that has occurred in that time frame.
Visualising how a group of quantities changes over time. Items are "stacked" in this type of graph allowing the user to add up the underlying data points.
Wikipedia (2012) Line chart. Retrieved from http://en.wikipedia.org/wiki/Line_chart
Wallace, R. (2005, May 16). Line graphs and scatter plots. Retrieved from http://www.ncsu.edu/labwrite/res/gh/gh-linegraph.html