A trendline is an additional line that indicates the slope (or trend) in a particular data series and is also known as a line of best fit. Trend lines can be helpful when you are analyzing data because they can forecast future values based upon your current data.
Follow the below steps to add Trendline in Excel:
Step 1: Select a Chart.
Step 2: Now, click on the CHART ELEMENTS (PLUS icon) in the upper right corner of the chart.
Step 3: From CHART ELEMENTS select the TRENDLINE option.
Step 4: You can FORMAT TRENDLINE also by further clicking on the trendline arrow.
Step 5: Now click on More Options… and can select any option from the list.
At this point, you have successfully added the Trendline on a chart in Excel