Dash by plotly is also amazing its like shiny but for python! - we were able to whip together an app that would let you drag and drop xyyy data and get a scatter plot instantly - you can try it here (first load takes 1-2s):
I'm not sure if there's a convent way in R Shiny to upload files using drag and drop, but otherwise this would be a dozen lines of code in R, at most. Shiny creates the plumbing and interactivity automatically.