Line Chart

A line chart also known as a line plot or a line graph, is a type of chart that displays information as a series of data points called ‘markers’ connected by straight line segments. Commonly used in different fields to represent correlation between variables it is similar to a scatterplot, except that the measurement points are ordered (typically by their x-axis value) and joined with straight line segments.

Quick details

What: Discover Change, Rank

Why: Visualize time series and data changes at a glance

History of Line Chart

Some of the earliest known line charts are generally credited to Francis Hauksbee, Nicolaus Samuel Cruquius, Johann Heinrich Lambert and William Playfair. Economist William Playfair, the author of “The Commercial and Political Atlas” (1786), used 43 line chart variants to explore time-series data about political and economic matters. Another use of a line chart appears shortly before Playfair in 1767, when a Swiss mathematician, physicist, philosopher, and astronomer Johann Heinrich Lambert used it regularly to illustrate his discoveries in physics.

Published in 1919, Dwiggins used this parody graph to express his opinion of standards in printing

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When to Use a Line Chart?

1When you need to make decisions by analyzing chronological trends

Use a line chart to visualize a trend in data over intervals of time – a time series – to get a picture of how your data changes over a continuous period of time. The line graph is hence often drawn chronologically, to discover the trend with time and adjust strategies according to the trend. The slopes between the markers can allow you to judge whether the trend is about to decrease or increase. The slope of the line segments if of interest, can be found out by generating mathematical formulas for individual segments.

Unlike scatter plots, the independent variable can be either scalar or ordinal. In the example above, Month could be thought of as either scalar or ordinal

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2When you need to clearly display relationships with your continuous periodical data

Use line graphs when you need to understand the underlying patterns that exist in the correlations between any two numerical values, how the change in one creates a change in the other. This makes a line graph as a powerful visual tool for marketing, finance, and other areas, also useful in laboratory research, weather monitoring, or any other function involving a correlation between two numerical values.

Stacked Line Chart displaying change in river discharge rate with respect to different months and years

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3When you need to know the rate of change of your data with respect to another variable

Use a line chart when you need conclusive evidence on whether the data is growing or decreasing, by calculating the slopes that connect the dots on a grid. Further, measurements such as the gradient or the area under the curve can be made visually, leading to more conclusions or results from the data table. Line charts can be used to show tendencies based on other continuous periodic values as temperature, distance, age, etc if the periods are equally distributed on the x-axis.

Spline Line Chart displaying change in slope with gradient colors

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Types of Line Chart

1. Spline  Chart

A chart used to show smooth gradual changes and is very similar to the Line Chart. It uses smooth curved lines and thus is also known as Smoothed / Curved Line Chart. 

2. Step Line chart

A chart where the lines are drawn in incremental steps rather than as straight lines between each value. These are especially useful when changes occur at certain times but the values remain more or less stable between changes.

3. Stacked lines chart

Chart with two (or more) independent and one dependent variable. If two or more lines are on the chart, it can be used as a comparison between them. 

 

When Not to Use a Line Chart?

1When you have fewer values to represent as label/nominal variables

Use bar charts instead of line charts if you only have a few values of one category at the same time intervals. Line charts can work really well for a high number of values or values with different time intervals. However, for fewer categorical labels, the bar charts will be slightly easier for readers to compare the differences between values. In addition, a bar chart communicates more intuitively that your values “fill up from zero every month” (or day, year, etc.), e.g. sales. While a line chart is more intuitive for numbers that increase or decrease continuously, e.g. unemployment or population.

2When you need multiple categories to be compared in one line chart

If you want to show how values differ in different categories, consider a (stacked) bar, column chart or split bars instead. Line charts with multiple lines for comparison can be used however intersecting lines make the chart visually difficult to comprehend. Use a line graph when you want to clearly see the rate of change (slope) between individual data points. 

3When your representation needs to emphasize the amount of change rather than time flow and rate of change

Although line charts can work to show how different categories stand to each other (e.g. to show that one category overtook another one) but if the sum of your categories is as important as the categories itself, consider an area chart instead. Also, consider an area chart if your audience should be quickly able to spot that the chart is about percentages that add up to 100%.

 

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