Gann Fans are based on the work of W. D. Gann who believed strongly in the use of geometric angles. He considered a movement of one price unit for each time unit to be ideal. On a chart for which these units have the same spacing (vertical and horizontal), this movement would follow a line drawn at a 45 degree angle. Gann considered this 1/1 line to be primary line in the fan. He also considered lines drawn at 1/8, 1/4, 1/3, 1/2, 2. 3. 4 and 8 times the rate of the primary line to be significant. Most charts do not have the spacing required to produce true Gann Angles. However, if you draw a line that represents equal changes in time and price units (as you define those units), the other lines drawn by ChartOverlay will have the rates of change defined by Gann.
The primary (1/1) line provides support in an uptrend and resistance in a downtrend. If it is breached, a major reversal may be occurring and the other lines, in succession, can be expected to offer support or resistance. See Speed Resistance Lines and Fibonacci Fans, which behave in a similar manner. See Disclaimer.
To draw a Gann Fan Lines you must draw a Trendline (press the 
button, or choose Trendlines from the menu). Gann Fans are an option associated with Trendlines.