Understanding and Designing Chart Scales

Designing chart scales is crucial to creating clear and effective data visualizations. Several factors must be considered when choosing the right scale for chart axes:


Understand the Data

Before selecting a scale, it’s important to thoroughly understand the data you’re working with. Identify the minimum and maximum values, the range of the data, and any distinct patterns or trends. This will help you choose a scale that highlights the most significant aspects of the data. 


Choose an Appropriate Scale Type

There are two main types of scales – linear and logarithmic. Linear scales have equal intervals between values, while logarithmic scales have intervals that increase exponentially. Generally, linear scales are the most suitable if your data has an even distribution. However, if your data spans multiple orders of magnitude, a logarithmic scale may be more appropriate to emphasize proportional differences.  

There are several types of chart scales used in data visualization, each serving a specific purpose. Here are some common types of chart scales:

  1. Linear scale: A linear scale, also known as an arithmetic scale, represents values on an axis with equal spacing between each tick mark. It is used when the data points have a consistent and uniform progression. A linear scale is suitable for representing data where the absolute numerical difference between values is important, such as measuring quantities or displaying continuous data.
    • If your data has an even distribution and does not span multiple orders of magnitude, a linear scale with single-division scale bars is generally appropriate. In this case, the intervals between values on the scale bars are equal. It allows for a straightforward interpretation of the data, as the differences between values are represented proportionally. Using single-division scale bars on a linear scale is particularly useful when the data values have a direct and consistent relationship. It ensures that the chart accurately represents the magnitude of the data points and enables viewers to make precise comparisons between them.
  2. Logarithmic scale: A logarithmic scale represents values on an axis based on logarithmic transformations. It compresses a wide range of values into a more compact representation. Logarithmic scales are useful when dealing with data that spans several orders of magnitude. They help in visualizing exponential or rapidly changing data, such as stock prices, population growth, or earthquake magnitudes.
  3. Normalized scale: A normalized scale is a method of representing data where values are adjusted or scaled to a common baseline or reference point. The purpose of a normalized scale is to allow for easier comparison and analysis of data points relative to each other, particularly when dealing with variables that have different units, ranges, or magnitudes.
  4. Proportional scale: A proportional scale represents values based on proportional relationships or ratios. It is often used in pie charts, where each segment’s size represents a proportion of the whole. Proportional scales are effective in visualizing parts-to-whole relationships or percentages.

Example: If you’re visualizing the number of COVID cases over time, a linear scale might be appropriate for showing the even distribution of daily cases. However, a logarithmic scale could emphasize these proportional differences if you want to highlight how much cases have grown and decreased over time. 


    Start from Zero

    As a best practice, you should consider starting your axis at zero whenever possible, especially for bar charts. This ensures that the heights of the bars accurately represent the values and prevents any misinterpretation of the data.

    Example: Imagine comparing the sales of three different products with a bar chart. If you start the y-axis at 50, the bars might appear to have much larger differences in sales than they actually do. Starting the axis at zero eliminates this misinterpretation and allows a more accurate product sales comparison.


    Choose the Right Measurement

    The choice between relative or absolute measurements in chart scale design depends on the specific context and the objectives of the visualization. Here are some considerations for when to use each type of measurement:

    • Relative measurements: Relative measurements are useful when the focus is on comparing values and understanding the relationships or proportions between data points.
    • Absolute measurements: Absolute measurements are appropriate when the emphasis is on the specific values or quantities themselves, and their absolute magnitude is critical for understanding the data.


    Consistent Axis Intervals 

    It’s important to maintain consistent intervals on an axis to avoid confusion or misinterpretation of the data. Don’t use irregular intervals or change the intervals partway through the chart.  

    Example: If you’re visualizing the average temperature over a year, you should have consistent intervals between data points representing each month. Using different intervals could distort the data and make it look like temperature changes vary significantly from month to month when they might not. 


    Avoid Overcrowding the Axis Labels

    When adding labels to an axis, make sure they are easy to read and not too crowded. Too many labels can make the chart appear cluttered and harder to interpret. 

    Example: If you’re visualizing movie sales for the past 20 years, you might not need to label every single year on your x-axis. Labeling every 5 years could be enough to provide context without overcrowding the axis.


    Emphasize Key Data Points

    Customize the scale to emphasize key data points that are relevant to your story. You can use reference lines, gridlines, or custom labels to draw attention to these points.

    Example: If you’re visualizing an annual report of company sales, highlighting a target sales goal or a record-breaking year could help emphasize key data points and provide additional insights for your audience.

    In conclusion, when choosing the right scale for chart axes, you need to analyze your data, select an appropriate scale type, start at zero, maintain consistency, avoid label overcrowding, and emphasize key data points.