Scale Settings for Trend Lines
High points and low points appear to line up better for trend lines when prices are displayed using a semi-log scale. This is especially true when long-term trend lines are being drawn or when there is a large change in price. Most charting programs allow users to set the scale as arithmetic or semi-log. An arithmetic scale displays incremental values (5,10,15,20,25,30) evenly as they move up the y-axis. A $10 movement in price will look the same from $10 to $20 or from $100 to $110. A semi-log scale displays incremental values in percentage terms as they move up the y-axis. A move from $10 to $20 is a 100% gain, and would appear to be a much larger than a move from $100 to $110, which is only a 10% gain.
In the case of EMC, there was a large price change over a long period of time. While there were not any false breaks below the uptrend line on the arithmetic scale, the rate of ascent appears smoother on the semi-log scale. EMC doubled three times in less than two years. On the semi-log scale, the trend line fits all the way up. On the arithmetic scale, three different trend lines were required to keep pace with the advance.
In the case of Amazon.com (AMZN), there were two false breaks above the downtrend line as the stock declined during 2000 and 2001. These false break outs could have led to premature buying as the stock continued to decline after each one. The stock lost 60% of its value three times over a two year period. The semi-log scale reflects the percentage loss evenly, and the downtrend line was never broken.