I have been meaning to post about some tools I found/developing but its been crazy. That being said I came across this free design tool while watching some GDC talks. Its really cool as it is not just a simple graphing tool. It is a graphing tool that allows you to simulate your game systems!
I used to use spreadsheets to plan out my systems but this is really handy as now I can simulate how the system will interact in real time instead of just guessing. If you are doing anything that is fairly system heavy this tool might prove really useful to you.