Hello Sascha,
I think it's better for you if I give you the advive on what to use and that you tray to do it instead of giving you the result already done, this way you will learn how to do it. Meanwhile, if you have doubts, ask them, for sure someone in this forum will help you.
First of all you have to read data from your text file.
Use the functions on the FileIo - For instance Read characters from file or read from spreadsheet file (this one has the disadvantega of only returning numbers in it's default version, you can open it and change it accordingly to the instructions you can find in the documentation of the vi - don't forget to save it with another name (like read from spreadsheet file string.vi) otherwise it will then return strings forever...).
Assuming now that you have already your data in an array of strings, pass it through a for loop (the array will then be indexed - in each iteration of the loop you will have one element of the array).
The next step, inside the for loop is converting the date info in a LabVIEW timestamp, you can do this with strings operations - llok a the scan from string function and the build a cluseter that can be wired to hte Data/Time to seconds function.
You have also to convert the value itself to a number - look at the string to number conversion group inside the string group.
You have to build a cluster with the timestamp value and the temperature value and then wire this cluster to the output of the for look.
Connect this output to a XY graph.
Enjoy,
Paulo