Reading a line chart

What is a line chart?

Line charts are one of the most commonly used chart types. A line chart is most often used to show how something changes over time, like the population of New Zealand. You may have heard this type of data or chart called a ‘time series’.

Line chart of the estimated New Zealand population. The line chart is increasing. It has two lines, one based on de facto population, one based on resident population. De facto population is everyone present in NZ, rather than living in NZ.

For this type of chart, the x-axis (that’s the horizontal one) will show time. It could be seconds, days, months, years, or even something like days since a certain event/threshold.

The y-axis (the vertical one) shows the thing you are measuring, like the number of people in New Zealand. Measures can be things like counts, rates, proportions, and more.

Read our article about the different types of measures.

Understanding a line chart

Before getting into specifics about line charts, we recommend you read our article on the basics of chart reading.

Once you’ve done that, we’re ready to look at some specifics of reading line charts.

The basics

Because line charts are all about how something changes over time, when we’re looking at a line chart, we tend to care less about an individual data point and more about the relationship between points.

When a value is higher than the previous value, the line slants upwards. When it’s smaller, it slants downwards. And when it’s the same, it’s flat.

This means we can understand the relationships between points by looking at two things: the shape of the line and the trend.

Look at the shape

The shape is what it sounds like: the physical appearance of the line.

Is it spiky, going up and down in a seemingly unpredictable fashion? Lines that are spiky suggest the measurement is affected by a range of different factors that can be hard to predict.

Line chart of price of bananas. The chart has a spiky line that doesn't align with any consistent time period.

Is it seasonal, with regular peaks and troughs in a pattern repeated over the time period? Lines that have regular peaks and troughs suggest that there is one or more factors that have a strong regualr impact on the measurement, like time of year.

Line chart of public transport patronage in Auckland which spikes from month to month but overall increases each year

Is it mostly consistent, with points that all seem to increase or decrease?

Line chart of estimated population of Māori in New Zealand. The line steadily slopes upward

Is it flat, with little change in either direction?

Line chart of public transport patronage in Wellington which shows bus, rail, and ferry. Each line is mostly flat

Is it curving upwards, increasing by a larger amount each time? Lines that curve upwards indicate rapid growth, suggesting that one or more factors may have changed and are having a strong impact.

Line chart of passenger arrivals at Auckland airport from China. The arrivals start low, and then gradually increase more and more each year, resulting in a chart that curves upwards. There are some years where it drops slightly, so it is not a perfect curve.

Look at the trend

The trend tells us, overall, whether the values are increasing or decreasing or staying the same. You can have a line chart that is really spiky in shape with lots of increases and decreases, but that overall trends upwards over time.

Line chart of public transport patronage in Auckland which spikes from month to month but overall increases each year

What next?

Now that you know how to read a line chart, you’re ready to use the 5 questions to understand the chart.