Speed up/add a spinner during the upload for the inspecting data plot

[gcroci2]

We need to speed up and/or handle better the waiting time in the inspecting data plot. With big files, it now takes ~20s for the data to be plotted.

I could investigate if it's possible to speed it up, or/and I can add a meaningful waiting message/spinner for the user before the data are plotted (but after they're uploaded).