The reputation graph has been broken for years:
Dragging over the area below will not do anything. Clicking/hovering the bars will also not trigger a popup.
It used to work but since several years ago it stopped working for me. From the comments here I have realized it is working for other users but not me. Therefore I have taken a look at it and I found out that since the page probably thinks I am on a mobile device (I have a touchscreen on Windows 8.1) it is only listening for touch events. (Based on my experience, the code/underlying library must has been written before touchscreen PC's became popular.)
I tested it and indeed, the touch events work. This also explains why it stopped working years ago because I have switched to a laptop with touchscreen.
Another user seems to have a similar issue but they couldn't figure out why. I am not able to confirm that the problem he was experiencing is also related to touchscreens.
Chrome 51, Windows 8.1.