How can I plot in matlab like this？

I have data of X and Y, but only Z of the red dot are knew, how can I plot more smoothly like this image?.

[darova]

What do you know about interp1() or spline()?

[Rik]

If you know the function but only need to determine the two parameters, you can use the fit function.

[wei zhou]

Thank you for answering!