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When to Use a Line Chart to Represent a Key Performance Indicator

The line chart is another commonly used visual representation of key performance indicators (KPIs).  They are easy to understand and easy to create, line charts feature prominently on most performance dashboards inside of KPI tracking software packages such as ezzykpi.com.

Similar to a bar chart, line charts feature an X and Y axis, each with ordered values.  The line chart represents data points with a series of markers and has straight lines connecting each marker.  It is commonly used to visualise values over time (called a time series), with the X axis representing the time that has passed.  Time series over a lengthy time range is a great way to visualize trends.  A word of caution, short term time series can be misleading.

When displaying a line chart, the frequency of measurement may vary and still return useful data.  For example, a businessperson may only record the number of sales that a shop records 2 or 3 times per week.  While a daily total would be more accurate, a line chart still allows the user to obtain a sense of which way the KPI is heading.

A line chart is also useful for showing a trend and can help predict what is going to happen in the future.  This makes it ideal for KPIs which impact future growth of a business — sales, website traffic, resource utilisation.  It will help a business understand the current trends and plan for the future.

Line charts can be used to display multiple values at the same time.  For example, you may wish to display your website’s traffic, sales and bounce rate on the same line chart to assess how much value you are getting from a new traffic source.  Care must be taken to only include relevant data on the same line chart to avoid misleading or confusing the user with clutter.

Value can be added to line charts by including goal lines, average lines or median lines.  For example, if a company has a specific sales target, a line can be included across the line chart to understand which days were above or below the target. Maximum and minimum value lines can also be added to make it easier to find the worst and best performing periods on a line chart.

Line charts that only use two markers are called slope charts.  They are useful when you wish to highlight a specific time period or when you want to show overall trends, instead of recent trends.

Line charts are prominent on the performance dashboards of analytics software like Google analytics.  They are the default chart for many important KPIs including:

•        Traffic overview (perhaps the most important metric in analytics)

•        Age demographics

•        Gender

•        User behaviour (new vs returning)

When to Use Line Charts
Whenever you are required to track changes over time and the trends within KPIs, consider a line chart.  Make sure you highlight important points in the chart including median, high/low points and average to help the user gain more value from the chart.

To further improve the value added by a line chart, some KPI tracking software gives users the ability to hover over markers and obtain more information.  The ability to drill down into the markers allows users to discover what is driving change in a KPI.

The Downside of Line Charts
When a user is quickly looking at line charts it can be easy to miss small trends in the data.  This can be compensated for by colouring in the area below the lines to add visual impact.

Because the timeline aspect is flexible, a line chart may cause someone to come to the wrong conclusion about what is really happening with a key performance indicator.  It is important to give users the ability to change the timeline and default to a reasonable timeline that shows overall trends instead of short-term fluctuations.