If you can get hold of the data set there are tools in
R to do this. I have Etopo1 as a GeoTIFF, I think it is the ice/cell one from here though I may have converted it myself from the binary format.
Read the data (possibly with reduced resolution), compute the slope and plot.
## orig dims, reduced 4-fold (choose divisor to suit your needs / system)
x <- readGDAL("Etopo1.tif", output.dim = c(10800, 21600)/4)
## convert to raster format for calculations
r <- raster(x)
g <- slopeAspect(r, out = "slope", unit = "degrees")
## plot histogram
I use readGDAL since I'm more familiar with it, but you can stick with raster as a wrapper around the rgdal stuff to handle reducing resolution and so on, and not require in memory use.
class : RasterLayer
dimensions : 2700, 5400, 14580000 (nrow, ncol, ncell)
resolution : 0.06666667, 0.06666667 (x, y)
extent : -180, 180, -90, 90 (xmin, xmax, ymin, ymax)
coord. ref. : +proj=longlat +ellps=WGS84 +datum=WGS84 +no_defs +towgs84=0,0,0
values : in memory
min value : 0
max value : 38.11677
?hist for more plotting options.