Display Trendlines in charts
You can display or hide trendlines in the current default bar, line, area, bubble, or scatter charts.
Trendlines, also known as lines of best fit or regression lines, graphically illustrate trends in data series and are commonly used when charting predictions. A trendline is typically a line or curve that connects or passes through two or more points in the series, showing a trend.
You can specify the following types of trendlines:

Linear
Use a linear trendline when your data increases or decreases along a straight line at a constant rate. For example, if your chart displays a steady increase in revenue by product line over time, you could use a linear trendline.

Exponential
Use an exponential trendline when your data values increase or decrease exponentially, or at an increasingly higher or lower rate. For example, if your chart displays an exponential increase in revenue by product line over time, you could use an exponential trendline.

Polynomial
Use a polynomial trendline when your data values both increase and decrease. For example, if your chart displays both increases and decreases in revenue by product line over time, you could use a polynomial trendline.

Logarithm or Natural Logarithm
Use a logarithmic trendline when your data values increase or decrease rapidly and then level out. For example, if your chart displays a rapid decrease in revenue by product line over time and then a plateau, you could use a logarithmic trendline.

Power
Use a power trendline when your data values increase or decrease in a curve at a steady rate. For example, if your chart displays a steady increase in revenue by product line over time and your data points will fit a curved line, you could use a power trendline.

Moving Average
Use a moving average trendline when your data values fluctuate and you want to smooth out the exceptions to see trends. For example, if your chart displays wild fluctuations in revenue by product line over time, but you know that some data points are exceptions, you could use a moving average trendline.
If you are not sure which trendline type to use, try each type to see which one best fits most of your data points. For example, a linear trendline will not fit most points on a scatter chart with widely spread data points.
Trendlines do not support color transparency.