Simple visualization of LD matrix

[xiangzhu]

Although not the main focus or output of LDshrink, it will be straightforward (and perhaps useful) to generate some simple visualization results similar to Fig. 1 in Wen & Stephens (2010).

Also point out in documentation/manuscript that some existing packages can be used for this task: e.g. https://cran.r-project.org/web/packages/corrplot/index.html