before sudo apt-get install gnuplot
I sudo apt-get build-dep gnuplot to resolve package dependencies.

~$ sudo apt-get build-dep gnuplot
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Note, selecting 'liblua5.1-0-dev' instead of 'liblua5.1-dev'
The following packages will be REMOVED:
  libgd2-xpm ubuntu-desktop
The following NEW packages will be installed:
  debhelper diffstat html2text intltool-debian libbsd-dev
  libcairo-script-interpreter2 libcairo2-dev libedit-dev libexpat1-dev
  libfontconfig1-dev libfreetype6-dev libgd2-noxpm libgd2-noxpm-dev
  libglib2.0-dev libjpeg62-dev liblua5.1-0-dev libncurses5-dev libpango1.0-dev
  libpixman-1-dev libpng12-dev libreadline-dev libreadline6-dev libtinfo-dev
  libwxbase2.8-dev libwxgtk2.8-dev libxcb-render0-dev libxcb-shm0-dev
  libxft-dev libxrender-dev po-debconf quilt texinfo wx2.8-headers
  x11proto-render-dev
0 upgraded, 34 newly installed, 2 to remove and 0 not upgraded.
Need to get 9,100 kB of archives.
After this operation, 37.8 MB of additional disk space will be used.

It says the "ubuntu-desktop" will be removed,
but "ubuntu-desktop" is important.
What's should I do?