How to interpolate between asynchronous data samples?|General Excel Questions & Answers|Excel Forum|My Online Training Hub

Hi experts,

I have a tough question (to me).

Introduction
From my Home Automation HC2, I imported CSV values from the internal temperature logging dataqbase into Excel 2013.
Column A = Unix time stamp, col. B = DeviceID, col. C = DeviceName, col. D = temperature in degrees Celsius.
Temperature sensors report new values asynchronously and are immediately stored in the HC2 logging database.
I separated the imported temperature value data to correponding columns per sensor, t_ambient, T1, T2, T3, T4,
created a Scatter Graph with Smooth Lines and set option to 'Connect data points with line'. See attached file.

Goal
I am interrested in the graphical result after subtracting t_ambient from T1 to T4.

Problem
As temperature values arrive asynchronously there are no full rows of data to subtract.
I need to fill the gaps (the cells marked yellow) by interpolated values. As there are hundreds of different gaps sizes,
manually contructing interpolatiion forrmulas is am impossible job.

Question
How use values when present and interpolate if absent in a generally easy to setup way (effectively filling the gaps)?

Best regards,
John