To add a regression line, you need a search with only one attribute and one measure. If your chart contains more than one attribute or measure, this option will not be visible. As soon as you modify the search to contain a single measure and attribute, the regression line option becomes available. To add a regression line to your chart, follow these steps:
While viewing your Chart, select the Display icon on the right side.
If it is not currently activated, toggle the Advanced menu on.
Check the box in the Regression line section to Display on chart. You'll see options to edit the line type, thickness, and color as well.
You can show regression lines on most types of charts, including bar, stacked bar, line, and bubble charts.