Refresh the histogram display
When
a histogram is read from a cache instead of the document’s current
state, the Cached Data Warning icon 
appears
in the Histogram palette. Histograms based on the image cache are
displayed faster and are based on a representative sampling of pixels
in the image. You can set the cache level (from 2 to
8) in the Performance preference.
To refresh the histogram so that it displays all
of the pixels of the original image in its current state, do one
of the following:-
Double-click anywhere in the histogram.
-
Click the Cached Data Warning icon
. -
Click the Uncached Refresh button
. -
Choose Uncached Refresh from the Histogram palette menu.
For information about cache level, see Histogram palette overview.