I’m currently using Koofr which provides seamless backup between devices. I’m sure it may not be as secure as Proton Drive but it works so well. Proton Drive support for Linux is a big want for me.

I have tried a few sample spreadsheets and am very impressed so far. The only thing I can’t access is the charts format editor. I can access selection of series and so on but there is no way to change chart colours - I have tried different browsers, OS and permissions but I only get the one editing tab. Is this not yet implemented (Lumo seems to think that it is) or am I doing something wrong?