• Open a pre-configured project
• Create an integer tag:  Project Tree  → Show all tags->  <add new>, Name = HMI_Tag_1, Data Type = Int.

• Place the Trend control on a screen: Open a screen; Drag and drop a Trend Control object from the tool box (Controls division) on the screen.

• Configure the Trend Control:
• Object Properties : General  →  Trend Areas  → [0] Trend Area  →  Trends  → [0] Trends  → Data Source Y  →  Source = HMI_Tag_1

• Create a log file: Project tree -> HMI -> Logs -> Data logs (tab) ->  < Add new>
• Log the tag: HMI tag -> Default tag table, select the tag, Logging Tag (window between tag table and properties), <Add New>
• Link the data log to the trend:
• General  →  Trend Areas  → [0] Trend Area  →  Trends  → [0] Trends  → Data Source Y  →  Source: Combobox = logging tags, "..."=  Logs, Data_Log_1, Logging Tag_1

TIPs:

• If you will access the log’s file different users as the programmer user, maybe you should be necessary to declare the log archive path on the project / WinCC Unified configuration. You can see the archive path on the WinCC Unified configuration in the step Archive Settings. You can set the archive path in your project in: project tree, runtime settings, storage system, Main database location for logging, storage medium = local, folder = storage path archive.
• For HMI devices, it should be necessary to define where the data log will be stored (SD card, USB stick). It is configured in: Runtime Settings -> Storage system