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A **semi-log graph** is a type of graph that uses a linear scale on the x-axis and a logarithmic scale on the y-axis.

We often use this type of graph when the values for the y variable have much greater variability compared to the values for the x variable.

This type of graph is particularly useful for visualizing **percentage change** of some variable over time.

The following examples show how to read semi-log graphs in practice.

**Example 1: Semi-Log Graph for Plant Growth**

Suppose a biologist wants to create a line chart to visualize the growth of a certain plant during a 20-week period.

She first creates the following line chart using a linear scale on the y-axis:

This chart is useful for visualizing the raw change in plant height from one week to the next.

However, she can use a **semi-log graph** to more easily visualize the percentage change in plant height from one week to the next:

Notice that the y-axis is measured on a logarithmic scale.

Using this graph, we can see that the percentage growth of the plant is quickest in the early weeks and then slows down dramatically in the later weeks.

**Example 2: Semi-Log Graph for Investment Growth**

Suppose an investor wants to create a line chart to visualize the growth of a certain investment during a 30-year period.

He first creates the following line chart using a linear scale on the y-axis:

This chart is useful for visualizing the raw change in the investment value from one year to the next.

However, he can use a **semi-log graph** to more easily visualize the percentage change in the investment value from one year to the next:

Notice that the y-axis is measured on a logarithmic scale.

Using this graph, we can see that the percentage growth of the investment is actually consistent from one year to the next.

**Additional Resources**

The following tutorials offer additional information on using log scales in plots:

How to Create a Semi-Log Graph in Excel

When Should You Use a Log Scale in Charts?