Graphviz not exporting as png

Question

I want to be able to run a command like this in Python to convert a dot file to a visual diagram.

os.system("dot -Tpng decision-tree.dot -o decision-tree.png")

However, when I run the code, I get this error:

sh: dot: command not found

I'm wondering if I have a graphviz problem. When I type $ brew install graphviz to check, I get this message:

Warning: graphviz 2.40.1 is already installed, it's just not linked.
You can use `brew link graphviz` to link this version.

But when I type $ brew link graphviz, I get this message:

Linking /usr/local/Cellar/graphviz/2.40.1...
Error: Could not symlink share/man/man3/cdt.3
/usr/local/share/man/man3 is not writable.

Is this the reason for my problem and how can I fix it? Based on my googling, I've tried uninstalling & reinstalling graphviz but I'm not having any luck.