example using substitute with mathematical expressions

somewhat lengthy but effective idiom for plotting mathematical
expressions with values taken from variables in the R session

.gitignore

@@ -15,3 +15,5 @@ cache/
 _source/figure/
 _source/cache/
 _source/__*
+.Rproj.user
+*.Rproj

_config.yml

@@ -60,6 +60,13 @@ authors:
     github: kbroman
     twitter: kwbroman
     bio: Department of Biostatistics and Medical Informatics, University of Wisconsin-Madison; research in statistical genetics; developer of R/qtl (for R)
+  kushey:
+    name: Kevin Ushey
+    email: [email protected]
+    homepage: http://cdrv.github.io
+    github: cdrv
+    twitter: kevin_ushey
+    bio: MSc., University of British Columbia; interested in statistical genetics and data visualization
 
 # The production_url is only used when full-domain names are needed
 # such as sitemap.txt 

_source/2013-04-08-mathematical-annotation-in-r.Rmd

@@ -1,7 +1,7 @@
 ---
 layout: post
 title: "Mathematical Annotation in R"
-author: [rcore, yulijia, kbroman]
+author: [rcore, yulijia, kbroman, kushey]
 categories: [Base Graphics]
 tags: [mathematical annotation, Greek letters]
 reviewer: [yihui]
@@ -33,3 +33,23 @@ plot(rnorm(100), rnorm(100),
   xlab = expression(hat(mu)[0]), ylab = expression(alpha^beta),
   main = expression(paste("Plot of ", alpha^beta, " versus ", hat(mu)[0])))
 ```
+
+Finally, if we want to include variables from an R session in mathematical 
+expressions, and substitute in their actual values, we can use `substitute`.
+
+```{r math-text-sub, fig.width=5, fig.height=4, tidy=FALSE}
+par( mar=c(4, 4, 2, 0.1) )
+x_mean <- 1.5
+x_sd <- 1.2
+hist( rnorm(100, x_mean, x_sd),
+      main=as.expression( substitute( 
+        expr=paste(
+          "Histogram of 100 samples from X ~ N(", x_mean, ", ", x_sd^2, ")"
+        ), 
+        env=list(
+          x_mean=x_mean, 
+          x_sd=x_sd
+        ) 
+      ) )
+)
+```