Line graph

From Business Intelligence

This Tufte-inspired line graph is a good example of minimizing non-data ink.	Multiset line chart depicting change in stocks. This line chart uses color subtly to highlight the the data rather than as a decoration. Google Finance	This line chart with period shading is more efficiently data dense than a regular line chart, since it also highlights periods. Excel charts designed by William Oswald
ManyEyes has an example of a line graph of the average GPA of 1000-8000 level courses. Stylistically, the graph is designed well, however, the data is not a time series and thus another visualization would have worked better, such as a bar chart.

Description

This is a visualization showing how a quantitative value changes over continuous intervals. The focus is less on the data points itself and more on the trend of the line curves. This is an ideal visualization for time series data. Use this to track a quantitative value's rise and fall over time.

Data requirements

Two data series (x and y). The series on the y-axis should have a natural ordering, such as years in a time series.

Availability

SSRS, Business Objects

Considerations

Line graphs should be used only when the x-axis' sequential values have a natural order, for instance, sequences of years. Line graphs should not be used for arbitrarily ordered values such as "Germany, France, USA".
Time series data (for which the line graph is ideal) should always adjust for inflation.
Use an accurate baseline with proportionate scale. The graphical representation should be proportional to the numbers in the data, and the scale should not change throughout the graphic.
Often the grid lines and ticks are not needed on a line graph. Try using very light gray lines or eliminate them altogether.

Related techniques

Multiset line chart: Also known as the multiline chart. A chart with several lines depicting several datasets, allowing for comparison. Using this method with an excessive number of datasets may make the chart hard to read. Example: Book and Journal Costs

Multi-y chart: Also known as the two axis column line chart or the dual axis chart. Two or more line charts with the same X-axis but different Y-values. For instance, the X-axis can show intervals while the Y-axis of the first chart shows fruit and the Y-axis of the second chart shows rainfall. The Y axes need to be labeled and to have the same X-axis and the data needs to be related somehow and useful to compare. An alternative to superimposing lines, which might distract from one another. This is also good for comparing two charts with vastly different scales to one another.

Stack graph: Similar to the line chart, where data points are connected to each other. However, the lines are filled with color and multiple datasets may be "stacked" on top of one another to show how the variables contribute to the total magnitude of the graph. This may be a stacked column or stacked bar chart.

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