I want to be able to save the output in PNG and have installed Graphviz. Still it complains saying Graphviz is not installed:
The output format 'output.png' is currently not available. Please install 'Graphviz' to have other output formats than 'dot' or 'vcg'.
As mentioned in comments by @KanwarG the solution is to install Graphvis from as a proper package and not using the python package found using pip.
If using a mac then a solution is to used Homebrew. If you do not have Homebrew then follow these instructions to install it. Then it is easy to install Graphviz using the following command.
brew install graphviz
If you are using ubuntu Debian the following command is likely to work.
apt-get install graphviz
Hope this helps other people trying to find a solution.