The intent of your question is not entirely clear, as evidenced by
different interpretations by different responders.

If you click on the the plotted data series, the formula bar will
display the range of data plotted. If you extend that range in the
formula bar, you can add support x-values to visually extrapolate the
trendline beyond the data. Note however that when the mouse cursor
floats over the graph, y-values will be displayed only for actual x-y
data points in the graph, not for interpolated/extrapolated points on
the trendline.

If you right click on the trendline, and select Format Trendline, you
can go to the Options tab and request that the equation be displayed.
If you format that equation to display in scientific notation with 14
decimal places (to ensure that coefficient values are not obscured
through rounding) then you can calculate y-values for any x-values that
you desire. Alternately, most trendlies can be recalculated using
LINEST or LOGEST.

Jerry

Samantha wrote:

I want to add data points to the scatter plot that are not part of the data
ie. add points to find their concentration on a standard curve.