You can use the "Display as % of stories" button just above the results plot which should account for the growth in the total submissions per day, or did you mean something else?
(I apologize, I cannot edit my previous comment from the client I'm currently using.) I'm not seeing that % option, though. I'd be happy to send you a screenshot somehow, if it helps.
That is odd sorry about that, you can email it to me "elioty at hotmail dot co dot uk" or stick it in a github issue: https://github.com/EliotJones/HnTrends
