Note: Trendlines are available only for certain chart types (including two-dimensional column, bar, and line charts). Furthermore, they should not be stacked.
If desired, you can have a trendline shown for each data series of the chart. For more information, see Showing/hiding the trendline.
Trendlines can be used to graphically display the trend of a data series (i.e., to what extent the data series rises/falls on average). Trendlines can also be extended beyond the given data points, so that they display a forecast for future values.
A statistical technique called regression analysis is used to calculate trendlines.
Changing properties: To change the properties of a trendline, select it, right-click it and choose the command Trendline: Properties from the context menu.
Tip: Alternatively, you can choose this command via the Edit properties icon 
on the ribbon tab Chart | group Chart elements. The other alternative is to simply double-click on the trendline.
You can make the following settings:
Format tab
▪Line section
| Here, you can change the appearance of the trendline. Auto uses a thick black standard line. None removes the line. The option Custom lets you customize the line style. |
Trendline tab
▪Type section
| Here, you can select the type of trendline to be displayed. In addition to linear trendlines (i.e. simple best-fit lines), trendlines such as exponential or polynomial trendlines can also be displayed. |
| The available types of trendlines are based on the following formulas: |
Linear: |
y = ax + b |
Polynomial: |
y = b + c1x + c2x2 + c3x3 + ... + c6x6 |
Exponential: |
y = cebx (e = Euler's number) |
Logarithmic: |
y = c ln(x) + b |
Potential: |
y = cxb |
Moving average: |
f(xi) = (xi + xi-1 + xi-2 + ... + xi-n+1) / n |
▪Based on series
| Here, you can specify the data series for which the trendline is to be displayed. |
▪Forecast section
| Here, you can extend the trendline beyond the existing data points. |
| If, for example, there are 3 data points (for example, the sales results for the 1st, 2nd and 3rd year), you can enter a 1 for Forward. The line is then extended by an additional period (1 data point), thus displaying a forecast for sales in the 4th year. |
▪Crossing point
| Here, you can force the trendline to intersect the Y axis at a specific Y coordinate. This option is available only for certain types of trendlines. |
▪Show formula in chart
| If this option is enabled, the formula used to calculate the trendline will be displayed in the chart. |
▪Show correlation in chart
| If this option is enabled, the correlation coefficient R2 of the data series will be displayed in the chart. |
▪Name section
| Here, you can change the name of the trendline if desired. This name is displayed in the legend. To do so, select the option Custom and enter the desired name. |