It should be just the X axis (XValueType property). Leave YValueType property on Auto.
A line from the log file looks like this: 14-Mar-18 18:22:56,14,31,12,29,13,27,16,23
The Date/Time stamp is common for all values and represents the X axis.
For a test, drop a new ChartBySampling control on the form and add your 8 addresses to PLCAddressValueItems collection.
Then go to Series and have it look like the attached picture - each series should only have the values circled in red changed from default.
You could possibly change the name of each series to reflect its PLC address.
Then run the program and see what you get.