lag function

[#1]

Hello

having issue implementing formula/script using lag function that will help populate a new column where original column has some missing data. so if rows 1-5 have x value and missing readings from 6-10 and a change in row 11 so rows 6-10 would be that x val

[#2]

If I am correctly interpreting what you want, Given a data table

The following formula will fill in the values in a new column as I believe you want them

If( Is Missing( :X ),
	Lag( :Y ),
	:X
)

[#3]

Yes! 

[#4]

Besides using Lag() if your data is fairly simple and you want to only fill missing values by repeating earlier value, JMP does provide interactive option "Replace Missing with Previous Value"