You need three or more equal-length columns of numeric or date/time data.
· If an x-, y-, or z-value is missing for an observation, Minitab does not plot the observation.
· If more than one z-value is observed for the same x-y coordinate, Minitab uses the mean of the z-values to calculate the contours.
More
For data subsetting options, see Subset. To create mesh data with Minitab, use Calc > Make Mesh Data.