You can save the histogram data to tweak it before plotting:
set.seed(12345)
x = rnorm(1000)
hist.data = hist(x, plot=F)
hist.data$counts = log10(hist.data$counts)
dev.new(width=4, height=4)
hist(x)
dev.new(width=4, height=4)
plot(hist.data, ylab='log10(Frequency)')
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