Live Data Graph will show date automatically in case the graph covers data for more than the current day. How many values are shown in "Live Data" is depending on the sensor's scanning interval and what's configured for the Live Graph in "Setup | System Administration | User Interface". When you scroll down, there you find section "Graphs" where you can configure how many values should be used for display in "Live Data".
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.