mean and variance plot for ggMarginal

[Manuel Reif]

Thank you for this great package!

Would it be possible to add a mean and variance plot (geom_pointrange) to type in the ggMarginal() function?
That yould be great.

Thank you!

[Dean Attali]

I'm not familiar with that type of plot. But I would be open for a PR if you'd like to tackle it