just now i have installed ubuntu 7. whether gnuplot or xemacs or xmgrace are already there or not, i typed
xmgrace
or
gnuplot
or xemacs
the msg comes:


gnuplot
The program 'gnuplot' is currently not installed. You can install it by typing:
sudo apt-get install gnuplot-nox
Make sure you have the 'universe' component enabled
bash: gnuplot: command not found


or

~$ xmgrace
The program 'xmgrace' is currently not installed. You can install it by typing:
sudo apt-get install grace
Make sure you have the 'universe' component enabled
bash: xmgrace: command not found


etc....
but when i do the same as per instruction it gives

# sudo apt-get install xemacs
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Couldn't find package xemacs



# sudo apt-get install xmgrace
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Couldn't find package xmgrace



please help me out

By default the 'universe' reposity (place on the internet where packages are held, that your package manager knows to look for them) is not enabled in Ubuntu, because by default they don't encourage you to use packages from there -- the packages in Universe aren't usually "open", for example they might not release their source code. You can use programs from that reposity if you want, but it's your own decicion; to enable that reposity, you need to edit (with a text editor) the reposity file(s) of apt and remove comment marks (#) from the beginning of the University reposity lines. See the file and you'll get the syntax. An easier method is to launch your graphical package manager (synaptic) and select "Manage reposities" (or equivalent) from the menu, and enable the Universe reposity. It might be called something else in the current Ubuntu, since I haven't used it (Synaptic) much; anyway it's in the package manager program and should be fairly easy to use. After enabling the reposity update package lists (either from Synaptic, or by 'sudo apt-get update') and re-try with the packages.