How do I interpolate missing data points in one data set so I can create an ensemble with multiple data sets?

I need to interpolate missing data points in an x,y data set (Age (ka), chemical value). I will need to interpolate chemical values between two given values at times that match with another data set. Ultimately, I will average out these values across multiple data sets in an ensemble.

What function will I need to use, or how would I go about the interpolation step. (If anyone has advice on how to average across multiple data sets, that would also be appreciated.) Thanks.