I noticed today when I was viewing my profile, looking at my reputation, that if you hover over a bar on your "reputation graph" the tooltip shows the date and the exact reputation earned like so:
However, if you have only earned a very small amount of reputation proportional to another day on your chart, the bar is very small (as it should be); except the "hoverable" area also becomes extremely small (and by extension hard to hover over). I suggest increasing the hover area to the size of that "entire day" regardless of reputation earned that day to increase usability:
If you hover anywhere in the red box the tooltip would appear. Obviously not a big issue but I think it would be useful.
EDIT: In response to Benjamin Gruenbaum discussing possible development overhead, I disagree. Here's a fiddle I put together in five minutes demonstrating this change: (observe the behavior of the left-most bar)