I would like to recommend you Staff Timer App
Staff Timer App is a web-based application which provides real-time monitoring, task tracking, and project management solutions to customers globally, working with remote or in-house teams.
Qalculate, a 'Powerful and easy to use calculator' that you find in all distros (even Windows) has the function you are looking for.
You can check it at: https://qalculate.github.io/
It's great for units too.
Hamster Time Tracker fulfills your requirements, except charting which can be done via export into your favourite spreadsheet app (e.g Excel).
Task Warrior or Time Warrior is another (easier to use on Windows).
Wikipedia's Comparison of time-tracking software lists many alternatives by Plattform and Features.
None of the previous answers suited my needs. XNote has the obnoxious millisecond timer with no way to change precision in the free version. I have found an open-source alternative LYZ which is riddled with fewer bs and is more aesthetically pleasing.