I think you can. KNIME is a platform that happily integrates all kinds of tools and code. You could also look into Java itself to do stuff.
The thing with the nodes for every step is deliberate. Our finance guys love it. For their reports and projections they can safely document and comment every step they make and it can be reviewed by other people without having to be a full blown programmer - although professionals are useful to plan and structure a program which most of the time is still necessary.
About your python question. Could you convert the time or duration variables into strings or doubles (days since …) and use that?
About graphics. KNIME has some great nodes there but sometimes I wish there was even more and I then revert to some python or R code that I copied and adapted from the web.
So you will have to plan ahead and as @kienerj said also think about the skill set in your team. A lot of companies build a broad know how cluster around KNIME with beginners, experts and a whole support community.