Compare the same four prices
Imagine a fictional stock observed at $25, $50, $100 and $200 at four equally spaced dates. Each step doubles the price. In dollars, however, the gains are $25, $50 and $100. A linear vertical axis makes the last step much taller than the first.
Equal dollar spacing
Read the chart data
| Observation | Price (USD per share) |
|---|---|
| 1 | 25 |
| 2 | 50 |
| 3 | 100 |
| 4 | 200 |
Read the percentage-spacing view
Equal percentage spacing
Read the chart data
| Observation | Price (USD per share · logarithmic) |
|---|---|
| 1 | 25 |
| 2 | 50 |
| 3 | 100 |
| 4 | 200 |
The log chart plots those same dollar labels with equal spacing for each doubling. The visual line now has a constant slope because each interval has the same proportional return. It has not become less profitable, less volatile or less risky simply because the line looks straighter.
A linear axis is often convenient for a narrow range where dollar distances matter. A logarithmic view can help compare proportional moves over a long period with large price changes. Neither is universally correct; choose deliberately and disclose the setting.
Check adjustments and valid values
Logarithms require positive values, so a standard log-price axis cannot display zero or negative prices. Comparing charts with different scales or split adjustments can create false visual differences. Confirm currency, observation interval and whether distributions are reinvested in the plotted series.
A 2-for-1 stock split approximately halves the per-share price and doubles share count at the event, without itself halving an owner’s value. A historical split-adjusted chart can remove that mechanical discontinuity. It does not automatically include dividends; check the data provider’s definition.
Use a chart-reading routine
Before drawing a trendline, read the axes and settings. Then calculate at least one return directly: (50 ÷ 25) − 1 = 100%, and (200 ÷ 100) − 1 = 100%. If equal returns look very different, the axis may explain why.
Do not compare steepness across unrelated charts with different sizes, date ranges or scales. State the numerical change and time interval in your notes. A visual trendline depends on the coordinate system; a return calculation makes that dependence visible.
Why does the final step look twice as tall as the middle step on the linear chart?
Show the explanation
The dollar gains are $100 and $50. Both represent a 100% return from their respective starting prices; a logarithmic axis gives equal percentage moves equal space.
Sources and editorial notes
Original explanations and hypothetical examples prepared with AI assistance. Primary reference check dates are listed below. Numerical examples are hypothetical unless explicitly identified otherwise.
- Fidelity: Three Tips for Setting Up Charts
Time-window and interval choices. Scale arithmetic, data series, chart walkthrough and failure case are original. Checked October 17, 2026.
- Charles Schwab: How to Read Stock Charts and Trading Patterns
Line, OHLC bar and candle representations; observation intervals; false signals and limits of technical analysis. Original settings exercises and failure rules are Academy examples. Checked October 16, 2026.
- Fidelity: Support and resistance
Support and resistance as historical demand/supply reference areas. Academy numerical zones, test rules and failure scenarios are original illustrations, not measured evidence. Checked October 16, 2026.