A function to facilitate `ggplot2`

graphs using a survey object.
It will initiate a ggplot and map survey weights to the
corresponding aesthetic.

`ggsurvey(design = NULL, mapping = NULL, ...)`

- design
A survey design object, usually created with

`survey::svydesign()`

- mapping
Default list of aesthetic mappings to use for plot, to be created with

`ggplot2::aes()`

.- ...
Other arguments passed on to methods. Not currently used.

A `ggplot2`

plot.

Graphs will be correct as long as only weights are required
to compute the graph. However, statistic or geometry requiring
correct variance computation (like `ggplot2::geom_smooth()`

) will
be statistically incorrect.

```
data(api, package = "survey")
dstrat <- survey::svydesign(
id = ~1, strata = ~stype,
weights = ~pw, data = apistrat,
fpc = ~fpc
)
ggsurvey(dstrat) +
ggplot2::aes(x = cnum, y = dnum) +
ggplot2::geom_count()
d <- as.data.frame(Titanic)
dw <- survey::svydesign(ids = ~1, weights = ~Freq, data = d)
ggsurvey(dw) +
ggplot2::aes(x = Class, fill = Survived) +
ggplot2::geom_bar(position = "fill")
```