Live data (i.e. data that is being transmitted to your application from the physical equipment in real time) can be plotted for any basic piece of I/O. For example, a well level can be viewed on the graph or table of the Historical Data Viewer page as its level rises and falls.
Historical data (i.e. data that has been collected and saved to your hard drive) can be plotted for any basic piece of I/O, as long as your VTS developer has created a special tag called a "logger" to record the historical data.
The distinction between live and file data plots lies in the time period configured for the data being plotted. If the time period for the graph is set to the present, then live data is plotted as it is read from the equipment processes composing your system. If the time period for the graph is set to the past, then historical data that has previously been logged or recorded to disk is plotted (again, a logger tag must exist in order for historical data to be plotted).
Information on live data plots is provided in Plot Data on the Historical Data Viewer Page.