pgfplots: problem with “non-smooth” intersection of two lines' ends

Question

I have a problem like the one in this question, concerning a tikz drawing but for a pgfplots plot. I want to create a plot that has one region filled without opacity and a neighboring one filled with opacity. The opaque region should be coated by a solid line. I managed to get pretty much what I want with this code:

\documentclass{standalone}

\usepackage{pgfplots}
\pgfplotsset{compat=1.3}

\usepackage{pgfplotstable} % For \pgfplotstableread
\usepgfplotslibrary{external}
\tikzexternalize

\usepackage{filecontents}

\begin{filecontents}{total.dos}
0  0  0  0  0   
1  2 -2  2 -2  
2  4 -4  4 -4  
3  2 -2  2 -2  
4  0  0  0  0 
5  1 -1  0  0 
6  2 -2  0  0 
7  1 -1  0  0 
8  0  0  0  0 
THIS WAS: TOTAL
\end{filecontents}

\pgfplotstableread[comment chars={T}]{total.dos}\total

\begin{document}

% Set a filename for the next tikzpicture.
\tikzsetnextfilename{total_dos} 

\begin{tikzpicture}
\begin{axis}[ymax=7, ymin=-7, xlabel={Energy [eV]}, ylabel={Intensity}, legend pos=north east]

\addplot [restrict x to domain=4:8, no markers, draw=red, fill=red, fill opacity=0.3] table [x=0, y expr=\thisrowno{2} - \thisrowno{4}] {\total}; \addlegendentry{$2-4$}

\addplot [restrict x to domain=4:8, no markers, draw=blue, fill=blue, fill opacity=0.3] table [x=0, y expr=\thisrowno{1} - \thisrowno{3}] {\total}; \addlegendentry{$1-3$}

\addplot [restrict x to domain=0:4, no markers, draw=blue , fill=blue] table [x=0, y=3] {\total}; \addlegendentry{$3$}

\addplot [restrict x to domain=0:4, no markers, draw=red, fill=red] table [x=0, y=4] {\total}; \addlegendentry{$4$}

\end{axis}
\end{tikzpicture}
\end{document}

This gives the following picture:

The problem is the intersections of the coating lines' ends (those that come from the draw=<color> options). They are not smooth, but look like this:

My question: How can I get these things smooth?

Side note: I know that using the option draw=none gives the behaviour I want, but then I have no line surrounding the opaque region.