Note that LabVIEW is a dataflow programming environment. Examine how you are setting up the dependencies in your block diagram between the plot, the DAQ VIs, the PID controls, and the loop(s).
Also, if you think this has to do with how you have set up your data acquisition, then you will have to describe your application in greater detail. For instance, what board and NI-DAQ version are you using? What version of LabVIEW are you using? What examples are you trying to follow? What exactly are you trying to do with the data acquisition values in correlation with the plot (i.e. when the input reaches X value, plot Y on the chart, and output Z signal, ...)?
Regards, Geneva L. Applications Engineer National Instruments http://www.ni.com/support