What is a spline chart?

In mathematics, a spline is a curve made up of two or move polynomial curves which are joined together. The curves are named after a spline used in ship building, which is a curved strip of timber that’s glued into a seam, often to repair a cracked plank. I. J. Schoenberg discovered and named mathematical splines in the 1940’s.

Splines are used in multiple ways now, especially for computer aided design (CAD) systems. But in the case of spline charts, in their simplest form they are curves, which are used on a chart instead of a straight line.

Why use a spline chart?

Basically, a spline chart is used because it looks more fluid than a normal line chart. It has a natural-looking curve to it and is quite aesthetically pleasing. Spline charts can be used to plot data that needs to suggest smooth, gradual changes—such as a product life cycle chart or an impulse-response chart. A spline chart is ideal in two main scenarios.

Following a time dependent variable

Almost anything that needs tracking over time can use a spline chart. Plot the time along the X-axis and variable of interest on the Y-axis to clearly show a journey. Spline charts can be used in almost any industry: retail, social science, manufacturing, and more.

If you need a chart that suggests smooth, gradual changes and tracks a trend over time, spline charts are perfect. In life, most actions do not happen immediately, and straight-line graphs can suggest that they do. A spline chart is far more accurate in suggesting a fluid, naturally occurring situation.

Finding and comparing patterns

Having multiple lines on a chart allows viewers to easily see differences between related variables and to identify patterns. An example of this is electronics sales over time in a number of different branches. Each branch will have its own spline, making it easy to see patterns such as seasonal variations and differences between branches.

While a line chart will plot these sales as a certain dollar amount reached on a certain day of the month, a spline chart gives more of an impression that the sales occurred gradually over the month. This is far more true to life, as sales will not happen all suddenly in one day.

Best practices when creating a spline chart

There are a number of standard practices to follow in order to create an effective and clear spline chart:

• Have a clear title so that people know what the graph is displaying
• X and Y axes labelled clearly
• Axes start at zero
• Scale is regular and even
• Use markers to plot the exact values that the curve goes through
• Choose how you want the line to be “smoothed”
• Use a strategy that does not misrepresent the data

If all basic practices are followed, the spline chart will be accurate, clear, and well-represented. This is important as the use of curves can distort information.

Advantages of a spline chart

Attractive appearance

The curves of the spline chart are unlike any other chart type and fluid in nature. They offer a level of aesthetics that aren’t found in charts usually and create a natural feeling.

Gives the impression of gradual change

The curves give the feeling that there has been a change gradual over time, rather than an instant change. This is ideal when plotting something like erosion levels of a river, seasonal plant growth, or something else based in nature.

Disadvantages of a Spline Chart

Can be cluttered

A multi-spline chart may be cluttered and confusing. Too many variables makes it difficult for you to see patterns and differences, which misses the point of the chart entirely.

Solution

Keep it simple. If there are any more than four variables, consider another chart type.

Curves give the impression of flexibility and can be deceptive

Spline charts are, by their nature, fluid looking. While all the same measurements are plotted like a line chart, spline charts use curves to join these points together to give an impression of fluidity. Generally, this is not an impression you want to give. However, if the chart is measuring water flows, weather, or other variables which are flexible, then a spline chart could be exceptionally accurate.

Solution

If measurables, exactness, and order is required, then a spline chart is not the best option. Try a regular line chart instead.

Variations to a spline chart

Spline area chart

Much the same as a line area chart, spline area charts map an area rather than having a single line. The mapped area is colored in, either with a solid color if there is one variable or translucent if there are multiple variables. These spline area charts can resemble mountains or rivers and are often used in geography to illustrate natural concepts or explain natural phenomena.

Multiple spline chart

Instead of having one fluid line represent the change in one variable over time, a multiple spline chart can graph a number of variables. While they all must be related and mapped on the same scale, this can be excellent at showing comparisons. Keep in mind, multiple spline charts can become crowded and confusing if too many variables are included.

Range spline area chart

Range spline area charts show two closely related variables mapped onto the same chart. The space between them is shaded in and resembles a river. Often, it will show the top and bottom of a range, such as maximum and minimum temperatures in a location over a year or blood pressure readings over a day.

Stacked spline area chart

Stacked spline area charts allow the viewer to see fluctuations of related variables, all at the same time. They are designed to show changes in qualitative data over time. They are different from a spline area chart as the data variables are more related, and they tend to mirror the same pattern

Alternatives to a spline chart

Line chart

The classic line chart is the obvious alternative to spline charts. Exactly the same principles but with straight lines. This avoids the problem where curves make figures look more fluid and flexible.

Area Chart

The area chart, and all its variations, can be used in place of an area spline chart or range area chart. These are similar to all spline charts except the lines are straight and not curved or smoothed. The exact same principles apply but produce less fluid-looking charts.

Future of spline charts

The use of spline charts is extremely narrow. This is due largely to their propensity to distort data. The use of curves is not suitable for all types of data. In general, line graphs are preferred, although use of dots along the curve at precise locations can help to avoid confusion or misleading data.