Two pending features to metricsgraphics

I’ve been slowly prodding the [metricsgraphics package](https://github.com/hrbrmstr/metricsgraphics/) towards a 1.0.0 release, but there are some rough edges that still need sorting out. One of them is the ability to handle passing in variables for the `x` & `y` accessor values (you _can_ pass in bare and quoted strings). This can now be achieved (in the `dev01` branch) via `mjs_plot_` and in `mjs_plot` proper in the github main branch thanks to a [PR](https://github.com/hrbrmstr/metricsgraphics/pull/31) by [Jonathan Owen](https://github.com/jrowen). If everything stays stable with the PR, I’ll just fold the code into `mjs_plot` for the `0.9.0` CRAN release.

One other pending feature is the ability to turn _basic_ (single `geom_`) `ggplot` objects into `metricsgraphics` plots. Sometimes it’s just easier/nicer to “think” in `ggplot` and it may be the case that one might have coded a quick histogram/scatter/line plot in `ggplot` and want an equally quick interactive version. This can also now be achieved (again, in beta) via `as_mjsplot`. While the previous addition is fairly self-explanatory, this new one needs a few examples. Please note that the package installation is coming from the `dev01` branch:

devtools::install_github("hrbrmstr/metricsgraphics", ref="dev01") 
 
library(metricsgraphics)
library(ggplot2)
 
dat <- data.frame(year=seq(1790, 1970, 10),
                  uspop=as.numeric(uspop))
 
set.seed(5689)
movies <- movies[sample(nrow(movies), 1000), ]
 
gg1 <- ggplot(dat, aes(x=year, y=uspop)) + geom_line()
gg2 <- ggplot(dat, aes(x=year, y=uspop)) + geom_point()
gg3 <- ggplot(movies, aes(rating)) + geom_histogram()
gg4 <- ggplot(movies, aes(rating)) + geom_histogram(binwidth = 0.1)
 
gg1
as_mjsplot(gg1)
 
gg2
as_mjsplot(gg2)
 
gg3
as_mjsplot(gg3)
 
gg4
as_mjsplot(gg4)

Which you can see below:

As you can see, `as_mjsplot` will do it’s best to figure out the bins (if using `geom_histogram`) and also axis labels. Support for converting `geom_vline` and `geom_hline` to markers and baselines (respectively) is a work in progress.

I’ve only done limited testing with some basic single `geom_` constructs, but if there are any bugs with it or feature requests (remember, the MetricsGraphics.js library has a very limited repertoire) please post an issue on GitHub tagging the `dev01` branch.

rud.is