Using positioning TikZ library, I am getting spurious white space between the arrow and the tikz node as shown in the figure below. It works fine with simple rectangles.
MWE:
\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{positioning}
\begin{document}
\begin{tikzpicture}
\node[rectangle,draw](node1){Node 1};
\node[rectangle,draw,below=of node1](node2){Node 2};
\node[below=of node2](tikzpic){%
\begin{tikzpicture}[scale=0.5]
\draw [step=0.25cm,lightgray,very thin] (0,0) grid (2.5,1.5);
\draw [->,ultra thick] (0,0.0) -- (0,1.5);
\draw [->,ultra thick] (0,0.0) -- (2.5,0);
\draw [fill=blue!7,ultra thick] (0.2,0) ..controls
(0.5,2.2) and (1.5,0.3) .. (2,0);
\end{tikzpicture}
};
\draw[->,ultra thick] (node1) -- (node2);
\draw[->,ultra thick] (node2) -- (tikzpic);
\end{tikzpicture}
\end{document}

