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 so that 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. By contrast, 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.
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.
RELATED FAQS

What are the best technical indicators that complement the Relative Vigor Index (RVI)?
Discover some of the best technical indicators that traders and analysts can employ to supplement the use of the relative ... 
What are the best technical indicators that complement the Relative Strength Index ...
Learn some of the best additional technical indicators that can be used along with the relative strength index to anticipate ... 
What is a common strategy traders implement when using the Relative Strength Index ...
Read about some of the many uses of the Relative Strength Index (RSI), and learn about basic strategies traders implement ... 
How are Rising Three Methods patterns interpreted by analysts and traders?
Understand the basics of the rising three methods candlestick pattern and how this strong continuation signal is interpreted ...

Intraday Momentum Index (IMI)
A technical indicator that combines aspects of candlestick analysis ... 
Forex Spread Betting
A category of spread betting that involves taking a bet on the ... 
Mass Index
A form of technical analysis that looks at the range between ... 
Money Flow Index  MFI
A momentum indicator that uses a stock’s price and volume to ... 
OnBalance Volume (OBV)
A momentum indicator that uses volume flow to predict changes ... 
Negative Volume Index  NVI
A technical indicator that relies on changes in a security’s ...