What is trend line
Trend or trend line as name depicts represents trend/slope of particular data series. Its also known as line of best fit as it can predict/forecast future behavior of data points in accordance to current data.
Different types of trend line
 Linear
 Logarithmic
 Polynomial
 Power
 Exponential
 Moving average
Linear trend line
Linear trend depicts the trend in data set in form of straight line. It can be said the linear trend is created on assumption that data will increase or decrease at steady rate. It should be used used where data changes at a stable rate.
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Logarithmic trend line
When data changes rapidly in positive or negative direction and then change in value stabilizes in such case logarithmic trend line should be used.
Polynomial trend line
Polynomial trend line is used when there is data set is fluctuating. When using polynomial trend line you have to specify order for trend line. The order 2 is used when data has one bend / peak. If data has one or two bends or peaks Order 3 is used and Order 4 is used for three or more bends or peaks.
Power trend line
Power trend line is used when data increases at fixed frequency, for example footfall in mall per minute in morning. Note: Power trend line cannot be created if your data contains values which are either zero or negative.
Exponential trend line
Exponential trend line is used when data increases or decreases at increasing rate. Note: Exponential trend line cannot be created if your data contains values which are either zero or negative.
Moving average
The moving average trend line is created by calculating the moving average of a certain number of values which is set by Period option.Note: Default period value is set to 2. Period value 2 means first point on trend line will be average of number 1 and 2 of data set. The second point of trend line will be average of number 2 and 3 of data set. Trend line created will show trends in data which not visible due to noise in data set.
Guide applies to office 2007 onwards
Forecast your data with trend line

 Arrange your data in a table and create a graph. We have created hypothetical weight growth chart of a baby. We have got 24 months data for weight series and 12 months data for actual series.
Now click on Insert tab and select line graph. Graph is ready  To add a trend line Right click on actual line. Select option add trend line.
 Format Trend line window comes up. Select Linear from available option.
Click on OK, trend line will appear on our data series line
 Arrange your data in a table and create a graph. We have created hypothetical weight growth chart of a baby. We have got 24 months data for weight series and 12 months data for actual series.