Calling of Function polyval:
The curve does not appear very smooth on this plot, but that is as there are only five points in the x vector.
To estimate the temperature at various times, polyval can be used for discrete x points; it does not have to be used with the whole x vector. For illustration, to interpolate between the given data points and estmate what the temperature was at 2:30 p.m., 2.5 would be used.
The polyval can also be used to extrapolate beyond the given data points, for illustration, to estimate the temperature at 1 p.m.: