How to install Gnuplot on mac 10.5.8?
http://brew.sh/

Install brew. The website lists the requirements, and the step by steps command line instructions. In short, it's:

ruby -e "$(curl -fsSL --insecure https://raw.github.com/mxcl/homebrew/go)"


Then:

brew install gnuplot


You're on 10.5.8 so installation requires '--insecure' as the older version of curl didn't have the CA secure certs packaged in.

Brew is also very handy for installing software in general. It's a unix package manager in the style of apt.