How to Make a Semi Log Graph in Excel

When you graph data with a massive range - like website traffic that starts at 100 visitors and grows to 100,000 - a standard chart can be pretty misleading. The initial growth gets flattened into a straight line at the bottom, making it impossible to see what really happened in the early days. This is where changing just one setting in Excel can give you a completely new perspective. This article will show you exactly how to make a semi-log graph in Excel and, just as importantly, explain when and why you should use one.

When (and Why) to Use a Semi-Log Graph

You should reach for a semi-log graph when you’re dealing with data that grows exponentially or covers several orders of magnitude. Instead of showing absolute values with even spacing (e.g., 10, 20, 30, 40), a logarithmic scale shows values by their multiples, with even spacing for powers of 10 (e.g., 1, 10, 100, 1000). This simple change powerfully visualizes rates of change, not just the raw numbers.

Let's use a common scenario: tracking the monthly user growth of a new app. In the beginning, it might jump from 50 to 150 users - a 200% increase. A year later, it might grow from 50,000 to 60,000 users - a 20% increase. On a standard (linear) chart, that early triple in users would be an invisible speck, while the later growth of 10,000 users would look like a huge mountain.

A semi-log graph fixes this. By compressing the scale for larger values, it visually highlights that the 200% growth in the beginning was actually far more significant as a rate of change than the 20% growth later.

Here are the best times to use a semi-log graph:

• Visualizing exponential growth: It turns an exponential curve into a straight line, making the consistent rate of growth immediately obvious. Think COVID-19 case charts, viral content spread, or compound interest.
• Comparing datasets with very different scales: Imagine you want to plot your website traffic against a massive competitor's. On a standard chart, your traffic would be a flat line at zero. A log scale puts both datasets on a comparable visual plane.
• Identifying acceleration or deceleration in growth: On a semi-log graph, a straight line means a steady percentage growth. If the line curves upward, your growth rate is speeding up. If it levels off, your growth is slowing down.

How to Make a Semi-Log Graph in Excel: A Step-by-Step Guide

Creating a semi-log graph in Excel is surprisingly simple. You're not building a new type of chart from scratch, you’re just changing the scale of one of the axes on a regular chart. Let's walk through it with a practical example.

Step 1: Prepare Your Data

First, make sure your data is organized in two columns. The first column should be your time interval (the X-axis), and the second should contain the values you want to measure (the Y-axis). Let’s say we’re tracking monthly website visitors for the past year.

Your data might look something like this:

Notice how the values grow from a few hundred to over fifty thousand. This is a perfect candidate for a semi-log graph.

Step 2: Insert a Standard Line or Bar Chart

Don't look for a "Logarithmic Chart" option right away. Start by creating a standard chart.

1. Highlight your entire data range, including the headers (e.g., cell A1 to B13 in our example).
2. Go to the Insert tab on Excel’s ribbon.
3. In the Charts section, choose a 2-D Line Chart or a 2-D Column Chart. A line chart is usually better for showing trends over time.

You’ll immediately get a chart that looks like a hockey stick. The first few months are barely visible, stuck flat against the bottom axis, while the last few months shoot up dramatically. This proves our point - a standard scale just doesn't tell the whole story here.

Step 3: Change the Vertical Axis to a Logarithmic Scale

Now for the key move. You need to tell Excel to change the vertical Y-axis from a linear to a logarithmic scale.

1. Move your cursor over the numbers on the vertical (Y) axis on the left side of your chart and right-click.
2. From the menu that pops up, select Format Axis....
3. This will open a “Format Axis” pane on the right side of your screen. Make sure you’re in the Axis Options tab (the icon that looks like three small vertical bars).
4. Look for the list of axis options and you’ll see a checkbox for Logarithmic scale. Click it.

That’s it! Your chart will instantly transform. The previously flat-looking data points for the early months will spread out, and the dramatic curve will straighten into a much clearer upward-sloping line.

Step 4: Customize and Refine Your Graph

Your semi-log graph is technically done, but a few small tweaks will make it much easier for others (and yourself) to read and understand.

Adjust the Base

In the "Format Axis" pane, right below the logarithmic scale checkbox, you'll see a field called Base. It defaults to 10. This is standard and works for most use cases, creating gridlines at 1, 10, 100, 1000, etc. Unless you have a specific scientific reason, it’s best to leave this at 10.

Set Axis Bounds

Sometimes Excel starts the log scale at a value like 0.1 or 1 when your data begins at 150. You can clean this up by setting a minimum bound. In the Axis Options, enter 100 into the "Minimum" field under Bounds. This starts the chart closer to your actual data and can make it look cleaner.

Add Gridlines

Reading a logarithmic scale can be tricky. Primary gridlines at each power of 10 are helpful, but minor gridlines can add even more context.

• Right-click the vertical axis again.
• Select Add Minor Gridlines. This will add fainter lines between 100 and 1,000, for example, helping the viewer better estimate the values.

Add Clear Labels

This is the most important final step. Because the axis isn't standard, you need to tell people what they’re looking at.

• Give your chart a clear title, like "Monthly Website Visitor Growth (Logarithmic Scale)."
• Label your axes. Click the chart, then click the green “+” icon that appears on the top right. Check the box for Axis Titles. Label the Y-axis something like "Website Visitors (Log Scale)." This prevents any confusion about why the numbers on the side are spaced so unusually.

Reading Your Semi-Log Graph: What to Look For

Now that you have your beautifully formatted semi-log graph, you can pull useful insights from it just by looking at the shape of the line.

• A perfect straight line: This is the classic sign of a constant rate of growth. This means your numbers are growing by the same percentage each period (e.g., traffic grew ~70% every month). On a linear scale, this would be a sharp, ever-steepening curve.
• An upward-curving line: If the line bends up on a log scale, your rate of percentage growth is accelerating. Not only are you growing, but you're growing at a faster and faster pace. This is the hallmark of true viral growth.
• A flattening line: If your line starts steep and then begins to level off, your rate of percentage growth is slowing down. Your numbers are still going up, but the monthly increase is becoming a smaller percentage of the total. This can be a sign of market saturation or a campaign running out of steam.

Final Thoughts

Switching a single axis in Excel to a logarithmic scale can completely change how you see your data, transforming a misleading graph into one that clearly shows growth rates and underlying trends. It elevates your data storytelling, allowing you to spot acceleration or saturation long before it would be apparent on a standard chart.

While mastering these sorts of charts in Excel is a valuable skill, we know toggling between spreadsheets and different reporting platforms is often just one part of the manual reporting grind. We created Graphed because we believe getting insights shouldn't require pulling reports from Google Analytics, Shopify, and your ad platforms, only to spend hours building charts. Instead of clicking through menus and formatting axes, you can simply ask for what you need - like, "show me website visitor growth this year as a line chart" - and get an interactive, real-time dashboard instantly.