The graph in the client area is definitely wrong. I just pushed a change to fix this, it should be live very soon. The graph in the control panel is accurate though.
As for “how is it possible that I used 4x the limit”, the reason is that there is some delay between when the usage is happening and the account actually being suspended. The system first needs to gather the data, check that the limit is reached, and then perform the suspension. During that delay, your website is still active and can generate additional server load, which is how you can exceed the limit.
We really didn’t cut everyone’s CPU quota by 75%.