multiple bquote items in legend of an R plot

问题:

Following works, (copy & paste into R)

a=123
plot(1,1)
legend('bottomleft',legend=bquote(theta == .(a)))

I want to have multiple items in the legend. All with greek letters. As a simple example, if I repeat the item twice the code does not work anymore

a=123
plot(1,1)
legend('bottomleft',legend=c(bquote(theta == .(a)),bquote(theta == .(a))))

I have tried many more complicated expressions but they all did not work.

Any help will be appreciated.

回答1:

In this case, plotmath is not able to coerce the list of calls to expressions.

> cs <- c(bquote(theta == .(a)),bquote(theta == .(a)))
> cs
[[1]]
theta == 123

[[2]]
theta == 123

> sapply(cs, class)
[1] "call" "call"

You can make this work if you coerce to expressions yourself:

> c(as.expression(bquote(theta == .(a))), as.expression(bquote(theta == .(a))))
expression(theta == 123, theta == 123)
> plot(1,1)
> legend('bottomleft',legend= c(as.expression(bquote(theta == .(a))), 
+                               as.expression(bquote(theta == .(a)))))

Another way is to coerce the original list of calls to expressions using sapply:

plot(1,1)
legend("bottomleft", 
       sapply(c(bquote(theta == .(a)), bquote(theta == .(a))), as.expression))

回答2:

To coerce the original list of calls to expressions it is not necessary to use sapply(). It suffices to use as.expression() only for one of the components within the c() construct:

plot(1,1)
legend("bottomleft", 
       c(as.expression(bquote(theta == .(a))), bquote(theta == .(a))))

c() will then automatically coerce the whole list, to the expression class.