So this is how the Graphs module works. It looks at the start datetime and the end datetime of the graph, divides it up by a maximum number of slots and then returns data points that are interpolated. This is why the ‘timestamp’ does not actually reflect the actual data.
Read this thread Timestamp best use. PHPFINA vs PHPTIMESERIES?
This may help as well Access data files & convert to csv
Another one From PHPFina to CSV?
If you search the forum you might find other topics that help you.