The entire chart is based on a logarithmic scale. A logarithmic scale measures price movements in percentage terms. An advance from 10 to 20 would represent an increase of 100%. An advance from 20 to 40 would also be 100%, as would an advance from 40 to 80. All three of these advances would appear as the same vertical distance on a logarithmic scale. Most charting programs refer to the logarithmic scale as a semi-log scale, because the time axis is still displayed arithmetically.
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