Problem with QPixmapCache

Hi,everyone!
I draw a large amount of point in pixmap. and then draw pixmap onto qgraphicsview which last was placed in a gridlayout. And when i resize the window, paintEvent is recalled which bring the repaint of large points on the pixmap. So i took QPixmapCache into account, and i use QPixmapCache::remove(key,pixmap) when there really need to repaint the pixmap such as point data changed, after drawing points on the pixmap ,and i use QPixmapCache::insert(key,pixmap) into the cache again. As we all know that QGraphicsview::paintEvent() is automatically recalled by Qt when you move mouse or use resizeEvent() , so as long as there's no need to repaint pixmap ,i just fetch the pixmap from the cache each time and the use drawPixmap(0,0, pixmap). And yes ,it really got what i need , it has enhance the speed.

but here came the problem, there only show a corner graphics of the pixmap whenever how i resize the window use resizeevent. but i have validated that i have draw all points in the pixmap and store it into the cache successfully when the data first come. Why i use resizeEvent, but the visible graphics of the pixmap hold in line with the same graphics when the wigdet is shown at first? Why? I'm sure that i have paint all the points in the pixmap completely. and i do can store and fetch from the cache.

Wish your help please. this problem has trouble me several days.Thank your very much!