I successfully drew a 3D grid graph from scattered data using the help of the GNUPLOT site:
http://t16web.lanl.gov/Kawano/gnuplot/plot3d2-e.html. It is indicated that the data values are averaged to create the mesh. Is it possible to obtain a 3D grid plot without averaging the data (as I would like to keep the raw values)?
Well, you'll have to make up your mind: either you want a grid plot, or you don't. If you do, then you have to have gridded data. You can let gnuplot grid them for you, which is what that site shows, or you can do it yourself. If you don't want to use gridded data, the plot can't be a grid plot.