I've noticed the gray bars on my twitter analytics.
Currently I'm averaging 200 impressions in the days that I don't tweet. :) Keep that in mind it's a number's game. The day I had 14 tweets, I had 485 impressions and the day with 8 tweets brought 562 impressions.
It's cumulative rather granular. So don't hold back, engage if you want to grow.
Share your stats if you like, it's a fun to look at other's and have an opinion about what's going on.