gnu: Add r-cowplot.

* gnu/packages/statistics.scm (r-cowplot): New variable.

gnu/packages/statistics.scm

@@ -4052,3 +4052,27 @@ (define-public r-compquadform
 quadratic forms in normal variables using Imhof's method, Davies's algorithm,
 Farebrother's algorithm or Liu et al.'s algorithm.")
     (license license:gpl2+)))
+
+(define-public r-cowplot
+  (package
+    (name "r-cowplot")
+    (version "0.7.0")
+    (source
+     (origin
+       (method url-fetch)
+       (uri (cran-uri "cowplot" version))
+       (sha256
+        (base32
+         "03iimcsh1pk7iqzjdlfcj43b8khijdk4hg00j4jdllv19xsfb0hx"))))
+    (build-system r-build-system)
+    (propagated-inputs
+     `(("r-ggplot2" ,r-ggplot2)
+       ("r-gtable" ,r-gtable)
+       ("r-plyr" ,r-plyr)))
+    (home-page "https://github.com/wilkelab/cowplot")
+    (synopsis "Streamlined plot theme and plot annotations for ggplot2")
+    (description
+     "This package provides some helpful extensions and modifications to the
+ggplot2 package to combine multiple ggplot2 plots into one and label them with
+letters, as is often required for scientific publications.")
+    (license license:gpl2)))