The interpretation of a stock chart can vary among different traders depending on the type of price scale used when viewing the data. As this question suggests, the two most common types of price scales are 1) logarithmic (also referred to as log) and 2) linear (also referred to as arithmetic).
A linear price scale is plotted on the side of the chart there is an equal distance between the prices, and each unit change on the chart is represented by the same vertical distance on the scale, regardless of what price level the asset is at when the change occurs.
A logarithmic price scale is plotted so that the prices in the scale are not positioned equidistantly; instead, the scale is plotted in such a way that two equal percent changes are plotted as the same vertical distance on the scale.
[Note: The difference between linear and logarithmic price scales is important to understand when reading charts, but there are many other forms of technical analysis that you can use to identify and capitalize on price trends. Investopedia's Technical Analysis course will teach you basic and advanced technical analysis skills with over five hours of ondemand video, exercises and interactive content.]
As you can see from the charts above, an increase in price from $10 to $15 is the same as an increase from $20 to $25 on the linear chart, because both scenarios represent an increase of $5. However, a logarithmic price scale will show the vertical distance between $10 and $15 to be different than the $5 distance between $20 to $25. The reason for this is that a change of $5 when the price is at $10 represents a 50% increase, while a move from $20 to $25 is a 25% increase. Since a 50% increase is more significant than 25%, chartists will use a larger distance between the prices to clearly show the magnitude of the changes.
When using a logarithmic scale, the vertical distance between the prices in the scale will be equal when the percent change between the values is the same. Using the above example, the distance between $10 and $15 would be equal to the distance between $20 and $30 because they both represent a price increase of 50%. In general, most traders and charting programs use the logarithmic scale, but it is always a good idea to explore other approaches to determine which is the most suitable for your trading style.
To learn more, see our Technical Analysis Tutorial.

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