Knitr chunk plots within a latex table - size issues

Question

I am writing a report within R studio using the knitr package. I am trying to succinctly describe a list of variables and there inclusion in an analysis. In order to do this I am using the long table package within latex. I have inserted a chunk into one of the table columns in order to display a histogram of some of the data. The issue is that this disrupts the width of the table column. I have tried setting the table width within the latex code but something is happening within the chunk that is over riding this.

\documentclass{article}

\usepackage{float}
\usepackage{longtable}

\begin{document}

 <<ch-req, echo=FALSE,results='hide',include=FALSE>>=
require(datasets)
 @

\begin{center}
\begin{longtable}{|c|p{.45\textwidth}|c|}
\caption{Summary table for variables in 'cars' data set} \label{tab:csumTab} \\

\hline \multicolumn{1}{|c|}{\textbf{Variable Original Name}} & \multicolumn{1}{c|}{\textbf{Variable Summary}} & \multicolumn{1}{c|}{\textbf{Renamed Variable}} \\ \hline\hline
\endfirsthead

\multicolumn{3}{c}%
{{\bfseries \tablename\ \thetable{} -- continued from previous page}} \\
\hline \multicolumn{1}{|c|}{\textbf{Variable Original Name}} &
\multicolumn{1}{c|}{\textbf{Variable Summary}} &
\multicolumn{1}{c|}{\textbf{Renamed Variable}} \\ \hline 
\endhead

\hline \multicolumn{3}{r}{{Continued on next page}} \\ 
\endfoot

\hline \hline
\endlastfoot
    `speed' & numeric   Speed (mph) of the car &  \\
                &
        \begin{figure}[H]
        <<histspeed1,echo=FALSE, fig=TRUE,out.width='2in', fig.align = 'left'>>=
        hist(cars$speed)
        @
        \end{figure}
    & \\
    \hline
    `dist' & This is the numeric stopping distance of the car (ft)& `Distance'\\
                   & \begin{tabular}{|c|c|c|}
                 \hline
                 min & median & max \\
                 \hline
                 2 & 36 & 120 \\
                 \hline
                 \end{tabular}& \\
  \end{longtable}
\end{center}


\begin{center}
\begin{longtable}{|c|p{.45\textwidth}|c|}
\caption{Summary table for variables in `cars' data set} \label{tab:csumTab} \\

\hline \multicolumn{1}{|c|}{\textbf{Variable Original Name}} & \multicolumn{1}{c|}{\textbf{Variable Summary}} & \multicolumn{1}{c|}{\textbf{Renamed Variable}} \\ \hline\hline
\endfirsthead

\multicolumn{3}{c}%
{{\bfseries \tablename\ \thetable{} -- continued from previous page}} \\
\hline \multicolumn{1}{|c|}{\textbf{Variable Original Name}} &
\multicolumn{1}{c|}{\textbf{Variable Summary}} &
\multicolumn{1}{c|}{\textbf{Renamed Variable}} \\ \hline 
\endhead

\hline \multicolumn{3}{r}{{Continued on next page}} \\ 
\endfoot

\hline \hline
\endlastfoot
    `speed' & numeric   Speed (mph) of the car &  \\
                &
        \begin{figure}[H]
        %<<histspeed2,echo=FALSE, fig=TRUE,out.width='2in', fig.align = 'left'>>=
        %hist(cars$speed)
        %@
        \end{figure}
    & \\
    \hline
    `dist' & This is the numeric stopping distance of the car (ft)& `Distance'\\
                   & \begin{tabular}{|c|c|c|}
                 \hline
                 min & median & max \\
                 \hline
                 2 & 36 & 120 \\
                 \hline
                 \end{tabular}& \\
  \end{longtable}
\end{center}

\end{document}

There are two tables just to show that the table size does work when the plot is not incorporated in the code. I have used out.width='2in' to change the size of the histogram in the table, but there is still a large area reserved for the plot by default(?). You can see this more clearly if you change fig.align = 'left' to fig.align = 'center'. I want the table to look how it does in the second example but with the plot incorporated.

Any help greatly appreciated.