Including charts in knitr where plot background has the same margin as figure width

knitr-ggplot2-flush-margin.Rnw

\documentclass[twocolumn, landscape]{article}
\usepackage{geometry}
\usepackage{float, placeins}

\addtolength{\columnsep}{25pt}

\usepackage{lipsum}

\title{Making charts in \LaTeX{} + \texttt{knitr} where $x = 0$ is on the left margin.}

\begin{document}
\maketitle
<<knitrOpts, echo=FALSE>>=
knitr::opts_chunk$set(echo = FALSE,
                      warning = FALSE,
                      message = FALSE,
                      fig.width = general_fig_width <- 7,
                      out.width = "7in",
                      fig.height = 4,
                      out.height = "4in",
                      results = "hide",
                      fig.show = "hide")
@

<<iris-plot, echo = FALSE>>=
library(ggplot2)
library(grid)
ggplot(diamonds) + 
  geom_point(aes(x = carat, y = price))

# For width identification
# grid.ls(view=TRUE,grob=FALSE)
# current.vpTree()
seekViewport('panel.3-4-3-4')
a <- convertWidth(unit(1,'npc'), 'inch', TRUE)


width_factor <-  general_fig_width / a
@
\lipsum
\begin{figure}[t]
\makebox[\columnwidth][r]{\includegraphics[width = \Sexpr{width_factor}\columnwidth]{figure/iris-plot-1}}
\caption{\Sexpr{paste0("The width of the panel is ", round(a, 2), " and the ratio used is ", round(width_factor,2))}}
\end{figure}
\lipsum


\end{document}