There is no built in functionality for this. You could use the (AutoShape)
drawing tool and draw a line, but that's not very satisfying. Or you could
determine the slope at the desired point, compute the endpoints of the line
with the calculated slope intersecting at the desired point, and add ax XY
series to the chart using these endpoints.

- Jon
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Jon Peltier, Microsoft Excel MVP
Tutorials and Custom Solutions
http://PeltierTech.com
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"Tangent line placement" <Tangent line
wrote in message ...

Drawing a Tangent Line on a Graph in Microsoft Excel

1. Create a scatter plot graph in Excel by selecting the data you want to plot and clicking on the "Insert" tab in the ribbon. Then, choose "Scatter" from the chart options.
2. Right-click on the data point where you want to draw the tangent line. Select "Add Trendline" from the drop-down menu.
3. In the "Format Trendline" window that appears, select the "Options" tab. Check the box next to "Display Equation on chart" and "Display R-squared value on chart."
4. Click on the "Line" tab and select "Linear" as the trendline type. Then, click "Close" to exit the window.
5. Right-click on the trendline and select "Format Trendline" again. In the "Line Style" tab, change the "Dash Type" to "Long Dash" or "Dash-Dot" to make the line more visible.
6. To draw the tangent line, use the equation displayed on the chart. The slope of the line is the coefficient of the x-term in the equation. Choose a point on the line and use the point-slope formula to find the equation of the tangent line. Then, plot the tangent line on the graph using the same x-values as the original data.

That's it! You now have a tangent line on your Excel graph.