combine png and tikz, scalingproblem

Question

due to the huge amount of data in my plot I wasn't able to compile the whole badboy at once after creating a tikz file and then using latex. Since the save_size won't go higher than 80000.

so while searching the internet I stumbled across another solution. I created a coordinate system and a colorbar as tikz without any plot via matlab. Afterwards I created the plot without any axis or so as a png picture.

The next would be to include the png graphic in my tikz file. So I did that but when I compile the whole tex file in which I \input my tikz file, the tikz picture and the plot picture won't align as I wish. I mean of course I can get my result after scaling and moving my included graphic but it is too time consuming.

\begin{axis}[%
width=4.1in,
height=3.80333333333333in,
view={0}{90},
%scale only axis,
xmin=0,
xmax=16,
xtick={ 4,  8, 12},
xlabel={x/H},
ymin=-5,
ymax=5,
ytick={-3,  0,  3},
ylabel={y/H},
zmin=-1,
zmax=1,
title={T},
colormap={mymap}{[1pt] rgb(0pt)=(1,1,1); rgb(64pt)=(1,1,0); rgb(160pt)=(1,0,0); rgb(255pt)=(0.0104167,0,0)},
colorbar,
point meta min=717.475533976793,
point meta max=2320.87848806079,
anchor=south west
]

\node (myfirstpic) at (0,0) [anchor=south west]{\includegraphics[width=4.1in,height=3.80333333333333in]{testplot_zsmngesetzt.png}};



\end{axis}

Especially what i dont get is why, even though my created axis and my included graphic are anchored south west, my graphic wont start at the same point as my axis

Do you have any idea how I can automatically align the two and let them start at the same point?

sincerely Mille