Calculate the density of samples along a 2-dimensional grid

```
bin2d(
x,
y,
n.bins = 100,
x.bin = NA,
y.bin = NA,
filter.frac = NA,
interpolate = TRUE
)
```

- x
n by m matrix where n is the number of observations and m is >= 1

- y
n by j matrix where n is the number of observations and j is >= 1

- n.bins
number bins over which to calculate intervals. Used to calculate x.bin if not provided.

- x.bin
vector of bin edges over which to bin.

- y.bin
vector of bin edges over which to bin.

- filter.frac
Used to beef up sampling for poorly sampled intervals. Interpolates intervals with less than filter.frac coverage.

- interpolate
use interpolation? T/F

A list with a matrix of density, x.bin and y.bin

Other gridding:
`kde2d()`

,
`quantile2d()`

Other bin:
`alignTimeseriesBin()`

,
`bin()`

,
`binEns()`

,
`binTs()`

Other plot help:
`AD2BP_trans()`

,
`BP2AD_trans()`

,
`axisLabel()`

,
`geoChronRPlotTheme()`

,
`getLegend()`

,
`getPlotRanges()`

,
`kde2d()`

,
`meltDistributionTable()`

,
`modeSelektor()`

,
`periodAnnotate()`

,
`quantile2d()`

,
`reverselog10_trans()`