I have a few points, graphed as a scatterplot and the points are connected by
a smooth line. (For example: (0,0), (5, 6), (6, 8), (10, 20).)

Since the points are connected by 3 different smooth lines, each line has a
different slope/degree. Is there any way to have Excel determine the y-value
of a point that is not a plotted point (anything besides x = 0, 5, 6, 10)?
For example, based on the smooth line, can I ask Excel to find the y-value of
x=1 or x=9?

Thank you very much for your help!