How to plot parabola in the selected segment using TikZ

Question

I want to plot this: area below parabola

I know, that I can find intersections using TikZ (tried \shade but don't know how to plot parabola only in a segment), but I don't know how to plot this R figure, and S+T, and U+V. How can I plot this? New detail: How [domain] works?

\tipc{[x=1cm,y=1cm]

  \def\xmin{0}
  \def\xmax{10}
  \def\ymin{0}
  \def\ymax{10}
  \draw[style=help lines, ystep=1, xstep=1] (\xmin,\ymin) grid
  (\xmax,\ymax);

  % axes
  \draw (-.25,-.25) node[auto] {0};
  \draw[->] (\xmin,\ymin) -- (\xmax,\ymin) node[right] {$Q$};
  \draw[->] (\xmin,\ymin) -- (\xmin,\ymax) node[above] {$P$};
  \draw[red] (2,2) parabola (8,8) node[right,black] {$S$};
  \draw[blue] (8,2) parabola (2,8) node[left,black] {$D$};
  \draw[dashed]  (5,0) node[below] {$q_A$} -- (5,3.5);
  \draw[dashed] (3.72,-0.5) -- (3.72,2.5) -- (6.27,2.5) -- (6.27,-0.5);
  \draw[<->] (3.72,-0.5) node [below] {$q_s$}  -- ++(2.55,0) node [midway,below] {$Im$}
  node [below] {$q_d$};
  \begin{scope}
        \draw[color=red!30,domain=1.72:3]
            (5,3.5) parabola  (2,8)  |- (3.72,3.5);
    \end{scope}
}

enter image description here

I guess a nice tutorial for your purpose is TikZ for economists. — Claudio Fiandrino, Jan 24 '13 at 10:17
Just from looking at it, i would say this would be much easier to do with pgfplots (if you have mathematical expressions for the curves). You can handle the problem of getting the intersection with the sugestions from http://tex.stackexchange.com/questions/21408/intersections-in-pgfplots — joão gândara, Jan 24 '13 at 10:50
I'm not sure if I've understand your question correctly... Do you want to create exactly the plot you've shown above? — Pouya, Jan 24 '13 at 11:52
@Ptech can you provide exact coordinates or and approximation of the plot would be ok? — Pouya, Jan 24 '13 at 11:56
@Pouya i know intersection coordinates, and i want to fill area below parabola with color. I don't know how to plot this "triangle" (because this triangle is below parabola) — Ptech, Jan 24 '13 at 12:01
@Ptech I know you wanted the parabola but i couldn't manage it. I post an answer with quadratic function. I'm sure you can create you own quadratic which are very near to tikz parabola. — Pouya, Jan 24 '13 at 13:25

score 7 · Accepted Answer · edited Apr 13 '17 at 12:35

Here is my solution. Please note that I have not used the parabola function of tikz because I failed to define the domain (not the end-points) and instead plotted two quadratic functions:

\documentclass{minimal}
\usepackage{tikz}

\begin{document}
    \begin{tikzpicture}

        \def\xmin{0}
        \def\xmax{10}
        \def\ymin{0}
        \def\ymax{10}
        \draw[style=help lines, ystep=1, xstep=1] (\xmin,\ymin) grid
        (\xmax,\ymax);

        \draw (-.25,-.25) node[auto] {0};
        \draw[->] (\xmin,\ymin) -- (\xmax,\ymin) node[right] {$Q$};
        \draw[->] (\xmin,\ymin) -- (\xmin,\ymax) node[above] {$P$};

        \def\intersectX{4.76}
        \def\intersectY{4.26}
        \def\QPX{4}
        \def\QPY{5}
        \draw[color=red] plot [domain=0:8] (\x,{((\x)^2)/10 +2)});
        \draw[color=blue] plot [domain=0:8] (\x,{((\x-14)^2)/20)});

        \fill[fill=pink,opacity=0.7] (0,\QPY) -- plot [domain=0:\QPY] (\x,{((\x-14)^2)/20)}) -- (\QPX,\QPY) -- cycle;
        \fill[fill=cyan,opacity=0.7] (0,\QPY) -- plot [domain=0:\QPX] (\x,{((\x)^2)/10 +2)}) -- (\QPX,\QPY) -- cycle;

        \draw [domain=\QPX:\intersectX] 
               plot(\x,{((\x-14)^2)/20)}) -- (\QPX,\QPY) -- (\QPX,\QPY) -- cycle; 

        \draw [fill=green,opacity=0.7,domain=\QPX:\intersectX] 
               plot(\x,{((\x)^2)/10 +2)}) -- (\QPX,\QPY) -- cycle;

        \draw[dashed]  (\intersectX,0) node[below] {$Q_1$} -- (\intersectX,\intersectY);
        \draw[dashed]  (0,\intersectY) node[below] {$P_1$} -- (\intersectX,\intersectY);
        \draw[dashed]  (0,\intersectY) node[below] {$P_1$} -- (\intersectX,\intersectY);
        \draw[dashed]  (\QPX,0) node[below] {$Q_2$} -- (\QPX,\QPY);
        \draw[dashed]  (0,\QPY) node[below] {$P_2$} -- (\QPX,\QPY);

    \end{tikzpicture}
\end{document}

Resulting in this:

enter image description here

I used some help from this answer as well.

This answer is great, I think, that i should get rid of parabola in tikz, and use plot instead (I had this question because parabola doesn't understand [domain]) — Ptech, Jan 24 '13 at 13:54

How to plot parabola in the selected segment using TikZ

1 Answers1