Aligning histograms and density plots

[teunbrand]

We currently have an example in the density documentation where this works out reasonably well by accident.
However if we pick flipper_len instead of bill_len we see misalignment happening.

VISUALISE flipper_len AS x FROM ggsql:penguins
DRAW histogram 
  REMAPPING density AS y
  SETTING opacity => 0.5
DRAW density
  SETTING opacity => 0.5

Currently the histogram density is computed as:

{count} * 1.0 / SUM({count})

To get good alignment with the density layer, we'd need to account for the binwidth as well.
I'm proposing the current histogram density variable becomes fraction or something, and have a new density computation that divides by the binwidth.

[thomasp85]

sounds good