Hi,
here you can find an example about how to draw a TGraph with time/dates on an axis: Time axis on TGraph
As for reading the values from the files, it depends if you want to use pyroot or the C++ root interface. You can perhaps be inspired by pages like http://www.cplusplus.com/forum/general/30918/ or https://python4astronomers.github.io/files/asciifiles.html .
Cheers,
D