I am trying to draw something similar to this:
This is what I have done so far:
\documentclass{article}
\usepackage{tikz}
\usepackage[active,tightpage]{preview}
\PreviewEnvironment{tikzpicture}
\setlength\PreviewBorder{5pt}
\begin{document}
\begin{tikzpicture}
\draw plot [smooth cycle] coordinates {(5,0.25) (6,0.35) (6.5, 0.2) (7,0.5) (7,1.65) (6.5,2.75) (5.8,2.75) (5.3,1.45) (4.8,0.85) } node at (6,1.7) {$X$};
\path[->] (3.1,1.7) edge [bend left] node[above] {$f$} (5.0,1.7);
\draw plot [smooth cycle] coordinates {(1.0,.1)(1.5,.2)(2.8,.5)(2.9,1.5)(2.8,2.8)(1.4,2.5)(0.5,0.5)} node at (1.8,1.8) {$U$};
\coordinate (O) at (0,0,0);
\draw[thick,->] (O) -- (1,0,0) node[anchor=north east]{$u_2$};
\draw[thick,->] (O) -- (0,1,0) node[anchor=north west]{$u_3$};
\draw[thick,->] (O) -- (0,0,1) node[anchor=south]{$u_1$};
\coordinate (B) at (4,0,0);
\draw[thick,->] (B) -- (5,0,0) node[anchor=north east]{$x_2$};
\draw[thick,->] (B) -- (4,1,0) node[anchor=north west]{$x_3$};
\draw[thick,->] (B) -- (4,0,1) node[anchor=south]{$x_1$};
\end{tikzpicture}
\end{document}
I didn't like the quality of the axes lines, nor the location of the axes labels. Any improvement on the drawing would be appreciated.


