# Negative mathematical expression coordinates

I only create this:

With this:

\documentclass{standalone}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
axis x line=bottom,
axis y line=left,
ultra thick,
y coord trafo/.code={\pgfmathparse{sqrt(#1)}},
y coord inv trafo/.code={\pgfmathparse{pow(#1,2)}}
]
(0,0)
(0.3,0)
(0.44,0.00016)
(0.45,0.00045)
(0.49,0.00071)
(0.5,0.001)
(0.51,0.00139)
(0.54,0.00176)
(0.56,0.00214)
(0.57,0.00243)
(0.61,0.00279)
(0.63,0.00327)
(0.64,0.00386)
(0.67,0.00433)
};
\end{axis}
\end{tikzpicture}
\end{document}


But I need put negative coordinates, for example: (-0.51,-0.00139)

also I need the red line and words (I,V):

• By setting axis lines=middle, both the axis will be placed in the middle, once you add negative coordinates.
• You can add labels using xlabel=$V$ and ylabel=$I$.
• Those labels can be placed at a different position with respect to the axis using x label style={at={(current axis.right of origin)}, anchor=west}.
• Drawing the red line can be done using \draw[red, dashed] (axis cs:0.6, 0)--(axis cs:0.6, 0.004);. Here I use axis cs coordinates, which are coordinates linked to your axis.
• Your last request is not possible. You can not add negative values on the y-axis, since you rescale the axis using a square root. The square root of a negative number is a complex number.

An example where I removed the y coord travo stuff and added some negative coordinates.

## MWE

\documentclass{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.14}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
axis lines=middle,
ultra thick,
xlabel= $V$, ylabel=$I$,
x label style={at={(current axis.right of origin)}, anchor=west},
%         y coord trafo/.code={\pgfmathparse{sqrt(#1)}},
%         y coord inv trafo/.code={\pgfmathparse{pow(#1,2)}},
]
(-0.48, -0.002)
(-0.43, -0.0002)
(-0.3, -0.0001)
(0,0)
(0.3,0)
(0.44,0.00016)
(0.45,0.00045)
(0.49,0.00071)
(0.5,0.001)
(0.51,0.00139)
(0.54,0.00176)
(0.56,0.00214)
(0.57,0.00243)
(0.61,0.00279)
(0.63,0.00327)
(0.64,0.00386)
(0.67,0.00433)
};
\draw[red, dashed] (axis cs:0.6, 0)--(axis cs:0.6, 0.004);
\end{axis}
\end{tikzpicture}
\end{document}


## Result

• Its perfect!!! Thank you very much for your help – Solid Oct 4 '16 at 21:53