Back in the days when XP was king, I used Octave 3.0 with GNUPlot, and AFAIK, it ran native, not in Cygwin (but that would be a fine way to go as well, I would think -- given the quirks of both *nix and Windows interfaces, that is). Worked fine, for what it was (i.e., not a very good MATLAB substitute, but usable). You should be able to render things with it, maybe using a command line script (or whatever language you like).
Should be plenty of plotting facilities in any of the common scripting/programming languages. Give or take compatibility in modern versions (you might need to use an old version; needless to say, an XP machine should be air gapped anyway, so the extra security risks of old versions shouldn't be making things worse for you). So, Java, Perl, PHP, Python and such.
And there's always Javascript. Ugly hack (gee, y'think?), you could do something roundabout but otherwise very portable: say, make a very basic server that dumps your real-time plotting data to a TCP port (this would be in C or C# or VB or whatever), then connect a socket to that in Javascript. The browser then handles all the rendering/formatting (via the many JS plotting libraries out there).
Tim