Sometimes when you're trying to plot datafiles (let's say measured data) against simulation results you get unexpected results.  This come from not selecting the right frequencies in the measurement.

When you add a measurement to a graph there is a default frequency selection but you can override this.  In the measurement dialog the field for this is at the bottom as shown here:

by pressing the ">" to the right of the Sweep Freq field you can see that for the data file there are two possible frequency selections for the file.

• Document is the list of frequencies in the actual datafile
• Project is the list of frequencies set in the project frequency list and those most likely being used by your schematic.

If you are trying to plot two different measurements against each other you'll want to make sure you have both measurements set to the same frequency range.

Another thing to watch for if you're using a datafile with a small number of frequency points in it is the interpolation method.  We have a knowledge base article that explains how to select an interpolation method and the tradeoffs to make.