[bbrizzi]

The first technique won't work if the sales are rounded off to the nearest pixel, which is pretty much almost always the case.

The second method won't work if the sales do not follow a Poisson distribution, for instance a product will have better sales at launch and during advertising campaigns. The sales figures don't always float around a constant average.

Also, I hadn't checked the math on the second method but the units don't add up.

On the LHS, you have [sales / pixel]x[pixels] = [sales]

On the RHS, you have ([sales / pixel]x[pixels])² = [sales]²

IIRC, the left hand side should be squared.

[sesqu]

No, the left hand should not be squared. It's a special property of the Poisson process that applies to the numbers, not the units.

[jules]

Both sides are unitless: sales is not in dollars but the number of sales.

[sesqu]

The units are actually rates: sales per day.

[gus_massa]

These methods also assume that the bottom level is 0. In this graph the bottom whirl is smaller, so probably it's a 0.