This might help.
http://en.wikipedia.org/wiki/Normal_distribution

I remember writing random test function for my Atari 8-bit to plot a normal distribution. I did this in high school after we learned about it and used pennies and graph paper to plot it. On the computer you would generate a random number in a range, say 1 - 10,
and as the numbers where generated you would count there frequency. on paper it would allow plot a bell curve on the computer both the Atari 8-bit and the PC, it would dip sharping in the middle to zero. I was think about writing another little program sometime to plot this again, for fun.