monitoring -> web, choose some scenario, choose "all" for the time period.
extend time period to the right (past the bar area) - that is allowed and eventually the scrollbar goes outside the browser window.
i believe that this got initially broken in r29459 - since that revision time period here can be expanded to the right, past the edge of the bar area, but as this page is still reloaded, the effect of the bug is negated.
then, r32021 exposed it by implementing graph reloading without page refresh.
reproducible in opera 12.14, firefox 18.0, konqueror 4.7.2