Caption above figure using knitr (LaTeX/PDF)

My preference tends to be to use LaTeX packages to achieve customisation like this: there is a large community on Tex StackExchange who have developed methods to load of problems similar to this.

The floatrow package can be used to reposition the caption above the figure. This is largely based on this previous answer.

Using R Markdown, as this is the most typically used workflow these days, the package can be loaded by including header-includes argument within the YAML, as follows:

---
output: pdf_document
header-includes:
   - \usepackage{floatrow}
   - \floatsetup[figure]{capposition=top}
---


```{r fig.cap="cap, cap, and cap"}
plot(1)
```

The output has the desired order with the code displayed first, followed by the caption then the plot.

If the code is not wanted, the echo=FALSE option can be added to the chunk header.

Try using hook:

<<include=FALSE>>=
f <- function(x, options) {
  paste("\\end{kframe}\n", 
        "\\caption{", options$capT, "}\n", 
        hook_plot_tex(x, options), 
        "\n\\begin{kframe}", sep = "")
}
knit_hooks$set(plot = f)
@

\begin{figure} 
<<a-plot, echo=TRUE, capT="cap, cap, and cap">>= 
plot(1) 
@ 
\end{figure} 

This is a slighty modified version of kohske's answer, that includes \begin{figure} and adds \label. Note however that it contains 5 lines, while the original code contains more than 150 lines, so it should be used in very limited settings.

f <- function(x, options) {
  lab <- paste0(options$fig.lp, options$label)
  paste("\\end{kframe}\n", 
        "\\begin{figure}\n\\caption{", options$capT, "}\\label{", lab,"}\n", 
        hook_plot_tex(x, options), 
        "\\end{figure}\n\n\\begin{kframe}", sep = "")
}
knit_hooks$set(plot = f)