change bandwidth approach in stat_density2d to match stat_density

[kendonB]

stat_density currently has the adjust parameter which defaults to 1 and scales whatever the default bandwidth would be. stat_density2d uses h, a length-2 vector which is directly passed to the underlying function in the units of the data.

[hadley]

I think this is a relatively straightforward fix if you have a little experience with geoms/stats:

• Add adjust parameter to stat_density_2d() and pass along in params. It would default to c(1, 1)

• Add adjust parameter to StatDensity2d$compute_group. It would affect the computation of h when it's NULL

[lock]

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