Google provided an ANSWER. Does this give you some ideas of what to do ?
Hi,
I'm not sure how to perform a certain task in Excel (or even if it's possible to do so).
I have acquired measurements and plotted the data as an XY-scatter to form a curve. I now need to choose a point on the curve and get Excel to draw the tangent line at that point and also to determine the slope. Obviously this is all experimental data, so I do not have an equation for the curve of which I could differentiate on my own (and I would prefer to not try to model the curve with an equation, although it could be a last resort).
I know more elaborate plotting software (like Logger Pro) is capable of drawing tangents to experimental curves, but I do not have access to such software. I was hoping there is a way of doing this in excel.
Can anyone help?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic