cumulative downloads

[thijsjanzen]

I think it would be really nice to add an extra tick-box that allows to display the cumulative downloads over time (cumsum), per selected package.

In fact, it seems that the code already allows for this, but that the user can not select that option (maybe something to select "cumulative" is broken in the ui?)

[Ax3man]

If you run the code here on github locally, the option is available. It seem the version at https://dgrtwo.shinyapps.io/cranview/ is simply out of date with this repo.