Hi Claus,
The "white block traversing from left to right" is perfmon showing you where it is up to. You may notice a hard red line on the left of the white block which shows the most recent sample is to the left of the red line. The white block makes it clear as the samples scroll across. You may also note that the time stamps change as the red line passes the time stamp intervals. You can change some settings in the perfmon properties dialog e.g.
If you are looking for longer term captures of performance data, may I suggest using the data collection sets (either system defined or user defined depending on your needs).
You can then review the data as you need.
HTH