I use qalculate too, although it's plotting UI/UX leave a lot to be desired, because as far as I understand it, it calls gnuplot, which opens a new window each time. UI is nice when using breeze on plasma, but it has a crude look on darwin

The greatest qalculate feature is it's integration with krunner on kde, which gives you access to a very powerful calculator / unit&currency converter with Alt+Space (by default), but it often doesn't trigger when you want to convert e.g. currency, so you need to force it by adding a prefix = to a query, like

    = 1 EUR -> PLN

Never used speedcrunch though