Align xtable caption left in knitr - r

Question

I have read a bunch of different posts on justifying xtable tables left but I cannot find details/work out how to make the caption justify left as well. Below is a reproducible example which justifies the table left but leaves the caption centred.
\documentclass{article}
\begin{document}
<<echo = F, results = "asis">>=
df = data.frame(x = c(1,2), y = c(4,6))
library(xtable)
print(xtable(df,digits=0, caption="Caption Left?"), include.colnames=TRUE, size = "small", comment=FALSE, latex.environments="flushleft")
#
\end{document}

Answer 1

I have found out how to do this. Simply import the LaTex Caption package and use the caption setup argument:
\captionsetup{justification = raggedright, singlelinecheck = false}
This will justify the caption to the left. The caption can be returned to its default centred position for additional tables or figures by repeating the function with the following modification before additional tables/figures.
\captionsetup{justification = centering, singlelinecheck = false}
The answered solution is:
\documentclass{article}
\usepackage{caption}
\begin{document}
\captionsetup{justification = raggedright, singlelinecheck = false}
<<echo = F, results = "asis">>=
df = data.frame(x = c(1,2), y = c(4,6))
library(xtable)
print(xtable(df,digits=0, caption="Caption Left?"),include.colnames=TRUE, size = "small", comment=FALSE, latex.environments="flushleft")
#
\end{document}
Which returns:

Answer 2

This operation is critical for anyone wishing to produce Markdown files in APA format since table captions are typically left-justified.
I'm going to clarify Robin's answer a little bit because, because totally new to the Markdown/Latex interface, it took me some time to figure out.
Upload the "caption" package (documentation available here) by including the
\usepackage{caption}
command in YAML (header) part of the document, preceded by
header-includes:
So an an entire YAML section at the header of the document might look like this:
---
title: "Supplementary Materials"
author: ""
date: "3/30/2018"
output:
pdf_document: default
editor_options:
chunk_output_type: inline
header-includes:
- \usepackage{caption}
---
Then, at any point in the document, before a code chunk (in its own white space), you can insert the code:
\captionsetup{justification = raggedright, singlelinecheck = false}
To change the settings back again, you can re-insert this code at any point in white space of the Markdown file (not in a code chunk!)
E.g.
\captionsetup{justification = centering, singlelinecheck = false}

Answer 3

If you don't mind using an alternative package (my own):
library(huxtable)
ht <- as_huxtable(df)
caption_pos(ht) <- "bottomleft"
print_latex(ht) # or just do `ht` if within a knitr or Rmd document