How do I use pchip only over a selected range?

I am creating a plot from a spreadsheet of data. I would like to interpolate values only over a specific range of this data. I will end up using all of the data in the spreadsheet but I would only like interpolate some of it. How do I go about doing this? Any example code would be very helpful, thanks.

[J. Alex Lee]

Do you know the criteria by which you will select these lucky few data points?

[Star Strider]

Context: Smoothing curve with duplicate values using pchip

I gave you example code in my Answer!