Gnuplot and GPS data.
1. The first step is to get your data into a format Gnuplot likes. The best way is tab delimited with three data feilds Latitude, Longitude and Altitude.
39.00653     84.14225     658
39.00651     84.14225     658
39.00651     84.14227     660
39.00649     84.14231     658
39.00649     84.14231     660
The data should be saved with the file extetion dat.
2. Fire up Gnuplot and run the commnds:
set nokey
set zrange [600:800]
set view 30, 45, 1.2, 1.2
splot 'yourdatafile.dat' with lines
This will show you a nice 3D view of your track log. Adjust the numbers in the 'set zrange' to your min and max elevations. You can adjust your view by changing the numbers in the 'set view' command. The plot will also show how bad the altitude can drift while standing in a single location.

3. So you want a nifty looking animation. Well thats easy too.

First save this script to a text file like spin.txt (Remember to change the apropreate numbers):
set nokey
set zrange [600:800]
set view b, a, 1.2, 1.2
splot 'yourdatafile.dat' with lines lw 3, 'yourdatafile.dat' with impulses lt 3
pause 0
if(a==360) a=0; b=b+10;
if(b<180) reread
Next fire up Gnuplot and type:
a=0;b=0;load 'spin.txt'