What is Logarithmic vs Linear?
A forex chart is usually drawn by converting price values into vertical positions. The two common choices are:
- Linear scale: the same numeric change in price produces the same visual change on the chart. If price rises from 1.1000 to 1.1200, that 0.0200 increase will look the same height as any other 0.0200 increase.
- Logarithmic (log) scale: the same percentage change in price produces the same visual change on the chart. For example, a move that is +2% will look similar whether it happens at a low price level or a higher one.
In practice, many charting tools offer a “linear” or “logarithmic” option for the vertical (price) axis. Both are valid ways to display the same underlying prices; they mainly differ in what the chart visually emphasizes.
How does Logarithmic vs Linear work?
Linear scale mechanics
On a linear axis, the chart’s vertical coordinate is proportional to the price level. That means:
- When the market price is higher, an absolute move (like +0.0050) can appear large in “percentage terms” even if the absolute size is the same.
- Trends can visually look more dominated by absolute moves, because vertical distance tracks the amount added to price.
Logarithmic scale mechanics
On a logarithmic axis, the chart’s vertical coordinate is proportional to the logarithm of the price level. A convenient way to think about the effect is:
- The chart compresses parts where price is growing quickly in absolute terms.
- The chart expands parts where price is lower and percentage changes are comparable.
This often makes it easier to compare relative growth or decline periods, because equal percentage moves correspond to equal vertical distances.
What changes when you switch scales?
Switching from linear to log usually changes the shape of lines and patterns drawn from the same data:
- A straight line on one scale may look curved on the other.
- Pullbacks and rallies may appear to have different “sizes,” depending on whether those moves are similar in absolute amount or similar in percentage amount.
Because of this, indicators or interpretations that rely on visual geometry (for example, how “steep” something looks) can change when you change the axis scale.
Relevant similarities and differences (comparison by criteria)
Visual meaning of distance
- Linear: distance corresponds to absolute price change.
- Log: distance corresponds to relative (percentage) price change.
Emphasis across price ranges
- Linear: emphasizes absolute movement, which can make higher price regions dominate the appearance.
- Log: emphasizes percentage movement, which can make different price regions feel more comparable.
Trend appearance and interpretation
- Linear: long runs can look consistently paced in absolute terms, even if percentage pacing changes.
- Log: percentage-based pacing tends to look more uniform when changes are similar in percentage terms.
Consistency when price levels differ
- Linear: equal absolute moves look the same even if their percentage effect differs.
- Log: equal percentage moves look the same even if their absolute effect differs.
Limitations and risks (what can go wrong)
Different questions, different answers
The biggest limitation is that the two scales answer different “how big?” questions:
- Linear helps when you care about absolute moves.
- Log helps when you care about relative moves.
If you interpret a chart visually as if it were using the other scale’s meaning, you can misunderstand what a move’s size represents.
Uncertainty from context changes
Forex prices can move through multiple regimes. A chart that looks supportive on one scale may look less supportive on another because the visual emphasis shifts. This is not necessarily “wrong” in either view; it is a reminder that your conclusion may depend on presentation choices.
Verification needs to be independent of one view
Because the scale affects geometry, it is sensible to treat scale choice as a source of uncertainty. When making conclusions from chart structure, verify that your reasoning still holds under the alternative scale, or check that your interpretation is supported by information that is not purely dependent on visual slope.
Practical constraints
Logarithmic axes require positive values because logarithms are only defined for positive numbers in standard charting contexts. Forex prices are typically positive, but specific instrument conventions or data transformations can still introduce edge cases in some platforms.
What you can independently verify
To reduce uncertainty, you can check the scale effect in a way that does not depend on forecasts:
- Take the same time window and plot it using both linear and log vertical axes.
- Compare how distances between the same price points change.
- Note whether the interpretation you would normally make depends on visual slope or spacing, which will shift across scales.
If your reasoning is truly about the underlying price behavior, it should be less sensitive to the axis choice than reasoning that depends on chart appearance alone.
Where advanced considerations typically show up
When users compare providers or charting tools, “log vs linear” can differ in how exactly the axis is implemented (for example, the way the tool labels ticks or how it handles rescaling after you change the visible range). Even with the same data, small implementation differences can change the appearance. For that reason, rely on concepts—absolute vs percentage distance—rather than on exact geometric look alone.
Quick guidance for choosing a scale (without predictions)
- If you want the chart to communicate absolute changes clearly, linear is aligned with that goal.
- If you want equal chart distance to correspond to equal percentage changes, logarithmic is aligned with that goal.
Either way, treat the scale as part of your interpretation framework and test whether your understanding remains consistent when the scale changes.