Which chart type is used when data requires a secondary axis?

• A. Bar Chart
• B. Line Chart
• C. Pie Chart
• D. Scatter Chart

Answer: (B)

When data requires a secondary axis, you want a chart that can clearly display two different scales side by side along a common dimension, like time. A line chart fits this needs well because it shows trends over that shared axis and easily accommodates a second vertical axis to plot another series with its own scale. This setup lets you compare patterns—such as peaks, troughs, and overall direction—without one series dominating the view due to scale differences.

Pie charts can't display multiple scales since they represent parts of a whole for a single dataset. Bar charts can be used for comparison, but they’re not the natural tool for dual scales on a continuous axis unless you’re using a more complex combo chart. Scatter charts focus on the relationship between two numerical variables and already rely on both axes, so they aren’t the typical method for adding a separate scale to a second series. So, the line chart is the go-to option when a secondary axis is needed.