# Drawing straight line between two points

I would like my code to give me something similar to the picture but my intersecting straight does not want to move above or below the variable 1 and when I try to get it look exact like my picture it shifts away from my coordinates.

\begin{figure}
\begin{center}
\begin{tikzpicture}[scale=3]
\draw[->] (-2,0) -- (2,0) node[right] {$R_{pe}$};
\draw[->] (0,-1.5) -- (0,2) node[above] {$R_{ph}$};
\draw[->] (-2,1) -- (2,1);
\draw[->] (1,-1.5) -- (1,2);
\draw[blue] (0,1)  -- (1,0) ;
\end{tikzpicture}
\end{center}
\end{figure}

• Please make your code compilable: it's much easier for people to help you, if you complete it with \documentclass{...}, the required \usepackage's, \begin{document}, and \end{document}. – Thruston Aug 27 '17 at 12:54
• Don't use \begin{center}...\end{center} inside figure or table environments. Just use \begin{figure}\centering...\end{figure} – user31729 Aug 27 '17 at 12:58

I don't really understand what your explanation of what goes wrong, but it's just a matter of choosing the proper coordinates I suppose. The slope of your blue line is -1, so you can for example start the line at (-1,2) instead of (0,1), and end it at (2,-1) instead of (1,0).

\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{figure}
\centering
\begin{tikzpicture}[scale=2]
\draw[->] (-2,0) -- (2,0) node[right] {$R_{pe}$};
\draw[->] (0,-1.5) -- (0,2) node[above] {$R_{ph}$};
\draw[->] (-2,1) -- (2,1);
\draw[->] (1,-1.5) -- (1,2);
\draw[blue] (-1,2)  -- (2,-1) node[right,black] {$R_0=\dots$};
\node at (0.25,0.25) {I};
\node at (0.75,0.75) {II};
\end{tikzpicture}
\end{figure}
\end{document}


Choosing the proper coordinates, as suggested by Torbjørn T.s answer is probably the best way to deal with your requirements. But if you don't insist on the blue line going exactly from the top of the vertical lines to the right of the horizontal lines, you can use shorten with negative lengths to extend the line.

Remark: In the example here I choose -3cm to better show the limitations, although -4cm would fit much better. And that length depens on the scale you're using.

\documentclass[border=1cm]{standalone}
\usepackage{tikz}
\usetikzlibrary{intersections}

\begin{document}
\begin{tikzpicture}[scale=3]
\draw[->] (-2,0) -- (2,0) node[right] {$R_{pe}$};
\draw[->] (0,-1.5) -- (0,2) node[above] {$R_{ph}$};
\draw[->] (-2,1) -- (2,1);
\draw[->] (1,-1.5) -- (1,2);
% -4cm for shorten is better, but set to -3cm to show the limitations
\draw[blue,shorten >=-3cm,shorten <=-3cm] (0,1)  -- (1,0) ;
\end{tikzpicture}
\end{document}


Edit: As Torbjørn T. mentioned, a downside to this is, that nodes can't be easily added to the end of the line. Using

\draw[blue,shorten >=-3cm,shorten <=-3cm] (0,1)  -- (1,0) node[right]{foo};


• A potential downside to this is node placement, try \draw[blue,shorten >=-3cm,shorten <=-3cm] (0,1) -- (1,0) node[right]{foo}; – Torbjørn T. Aug 27 '17 at 15:35