Graph Reputation
does not graph the reputation of someone, that would be silly (way too obvious). Instead it updates the column of reputation points earned numbers on the right of the graph, and it changes the highlighted area on the graph.
If you use the mouse to click and highlight a time period, the button will do absolutely nothing. But if you manually enter the dates below the graph, the button will update the highlighted area and the points (a twofer, if you will)!...... (although, simply changing the focus away from the date entry text boxes will also achieve the same effect, fwiw...)
(If you have Javascript turned off, the button will also do absolutely nothing, but this is ok, since neither the reputation graph nor the points earned will be visible at all.)