[Tex/LaTex] How to add a title to tikzpicture + smoother function lines

tikz-pgf

I know that my first question from the title has been asked several times, but when I try to apply the solutions from other the questions, they don't work.

What I want to achieve, is to add a title to these two functions:

\documentclass[12pt]{article}
\usepackage{tikz}
\begin{document}
\begin{figure}
\centering
\begin{tikzpicture}[thick,font=\footnotesize]
\fill [blue!10] (0,0) -- (0.55,0) -- (0.55,0.55) -- (0,0.55) -- cycle;
\draw[->] (0,0) -- (5,0) node[right] {$Quantity$};
\draw[->] (0,0) -- (0,5) node[above] {$Price$};
\coordinate (p1) at (0.55,0.55);
\draw[color=blue,domain=0.067:4.5] plot (\x,{(0.3)/(\x)});
\draw[dotted] (p1) -- (0.55,0) node[below] {$Q^*$};
\draw[dotted] (p1) -- (0,0.55) node[left] {$P^*$};
\fill[blue] (p1) circle (2pt);
\end{tikzpicture}
\begin{tikzpicture}[thick,font=\footnotesize]
\fill [blue!10] (0,0) -- (1,0) -- (1,2) -- (0,2) -- cycle;
\draw[->] (0,0) -- (5,0) node[right] {$Quantity$};
\draw[->] (0,0) -- (0,5) node[above] {$Price$};
\coordinate (p1) at (1,2);
\draw[color=blue,domain=0:2] plot (\x,{(2-\x)*2});
\draw[dotted] (p1) -- (1,0) node[below] {$Q^*$};
\draw[dotted] (p1) -- (0,2) node[left] {$P^*$};
\fill[blue] (p1) circle (2pt);
\end{tikzpicture}
\caption{Optimal side A and B pricing under marginal costs, based on XXX}
\end{figure}
\end{document}

Most solutions state that one should add \node + label to each of the tikzpictures, but I'm getting critical errors whenever I try to incorporate this. So my questions would be:

1) How can I add a title above each picture?

2) Regarding the convex curve in the left picture: there is an "edge" on the blue function – Any idea how this can be fixed?

Thank you!

Best Answer

One way would be to place a node relative to the current bounding box, as in the example below. You could of course use explicit coordinates as well, e.g. \node at (2.5,5.2) {Some title};.

For the plot, add samples=100 or something like that. The default number of samples is 25, more samples gives smoother curves (and longer compile times). And an unrelated note, don't write Price and Quantity in math mode.

\documentclass[12pt]{article}
\usepackage{tikz}
\begin{document}
\begin{figure}
\centering
\begin{tikzpicture}[thick,font=\footnotesize]
\fill [blue!10] (0,0) -- (0.55,0) -- (0.55,0.55) -- (0,0.55) -- cycle;
\draw[->] (0,0) -- (5,0) node[right] {Quantity};
\draw[->] (0,0) -- (0,5) node[above] {Price};
\coordinate (p1) at (0.55,0.55);
\draw[color=blue,domain=0.067:4.5,samples=100] plot (\x,{(0.3)/(\x)});
\draw[dotted] (p1) -- (0.55,0) node[below] {$Q^*$};
\draw[dotted] (p1) -- (0,0.55) node[left] {$P^*$};
\fill[blue] (p1) circle (2pt);
\node[above,font=\large\bfseries] at (current bounding box.north) {Some title};
\end{tikzpicture}%
\begin{tikzpicture}[thick,font=\footnotesize]
\fill [blue!10] (0,0) -- (1,0) -- (1,2) -- (0,2) -- cycle;
\draw[->] (0,0) -- (5,0) node[right] {Quantity};
\draw[->] (0,0) -- (0,5) node[above] {Price};
\coordinate (p1) at (1,2);
\draw[color=blue,domain=0:2] plot (\x,{(2-\x)*2});
\draw[dotted] (p1) -- (1,0) node[below] {$Q^*$};
\draw[dotted] (p1) -- (0,2) node[left] {$P^*$};
\fill[blue] (p1) circle (2pt);
\node[above,font=\large\bfseries] at (current bounding box.north) {Some other title};
\end{tikzpicture}
\caption{Optimal side A and B pricing under marginal costs, based on}
\end{figure}
\end{document}

Related Solutions

[Tex/LaTex] Rotate a node but not its content: the case of the ellipse decoration

The source of the difficulty is that ellipses are constructed in a particular way in TikZ. They are paths that start from the x-axis and proceed counter-clockwise around their centre. The vast majority of the time, the exact parametrisation doesn't matter. You appear to have found the one situation where it does!

In the actual question, you only want to be able to mirror the ellipse, and so draw it starting from the negative x-axis (the title of the question suggests a more flexible approach). That's actually not too hard since we can exploit the symmetry of the ellipse. The key is to provide it with a negative x-radius, since then it will start from the negative x-axis (and proceed clockwise, but we could correct for that by negating the y-radius as well). To do this, we interrupt the call from the node shape to the drawing command and flip the sign of the x-radius. The simplest way to do this is to redefine the \pgfpathellipse macro to do the negation and then call the original macro. The following code does this.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations,shapes,decorations.markings}
\makeatletter
\let\origpgfpathellipse=\pgfpathellipse
\def\revpgfpathellipse#1#2#3{%
  #2%
  \pgf@xa=-\pgf@x
  \origpgfpathellipse{#1}{\pgfqpoint{\pgf@xa}{0pt}}{#3}}
\makeatother

\tikzset{
  reversed ellipse/.style={
    ellipse,
    reverse the ellipse%
  },
  reverse the ellipse/.code={
    \let\pgfpathellipse=\revpgfpathellipse
  }
}
\begin{document}
\begin{tikzpicture}
\node[ellipse,
  draw,
  postaction={
    decorate,
    decoration={
      markings,
      mark=at position 1 with {
        \arrow[line width=5pt,blue]{>}
      }
    }
  }
] at (0,0) {hello world};

\node[reversed ellipse,
  draw,
  postaction={
    decorate,
    decoration={
      markings,
      mark=at position 1 with {
        \arrow[line width=5pt,blue]{>}
      }
    }
  }
] at (0,-2) {hello world};
\end{tikzpicture}

\end{document}

Here's the result:

rotated ellipse

(the arrow got clipped, but you can see where it lies)

[Tex/LaTex] TikZ: Drawing an arc from an intersection to an intersection

A solution which allows to draw intersection segments of any two intersections is available as tikz library fillbetween.

This library works as general purpose tikz library, but it is shipped with pgfplots and you need to load pgfplots in order to make it work:

\documentclass{standalone}

\usepackage{tikz}
\usepackage{pgfplots}
\usetikzlibrary{fillbetween}

\begin{document}

\begin{tikzpicture}

\draw [name path=red,red] (120:1.06) circle (1.9);
%\draw [name path=yellow,yellow] (0:1.06) circle (2.12);
\draw [name path=green,green!50!black] (0:0.77) circle (2.41);
\draw [name path=blue,blue] (0:0) circle (1.06);

% substitute this temp path by `\path` to make it invisible:
\draw[name path=temp1, intersection segments={of=red and blue,sequence=L1}];
\draw[red,-stealth,ultra thick, intersection segments={of=temp1 and green,sequence=L3}];

\end{tikzpicture}

\end{document}

enter image description here

The key intersection segments is described in all detail in the pgfplots reference manual section "5.6.6 Intersection Segment Recombination"; the key idea in this case is to

create a temporary path temp1 which is the first intersection segment of red and blue, more precisely, it is the first intersection segment in the Left argument in red and blue : red. This path is drawn as thin black path. Substitute its \draw statement by \path to make it invisible.
Compute the desired intersection segment by intersecting temp1 and green and use the correct intersection segment. By trial and error I figured that it is the third segment of path temp1 which is written as L3 (L = left argument in temp1 and green and 3 means third segment of that path).

The argument involves some trial and error because fillbetween is unaware of the fact that end and startpoint are connected -- and we as end users do not see start and end point.

Note that you can connect these path segments with other paths. If such an intersection segment should be the continuation of another path, use -- as before the first argument in sequence. This allows to fill paths segments:

\documentclass{standalone}

\usepackage{tikz}
\usepackage{pgfplots}
\usetikzlibrary{fillbetween}

\begin{document}

\begin{tikzpicture}

\draw [name path=red,red] (120:1.06) circle (1.9);
%\draw [name path=yellow,yellow] (0:1.06) circle (2.12);
\draw [name path=green,green!50!black] (0:0.77) circle (2.41);
\draw [name path=blue,blue] (0:0) circle (1.06);

% substitute this temp path by `\path` to make it invisible:
\draw[name path=temp1, intersection segments={of=red and blue,sequence=L1}];
\draw[red,fill=blue,-stealth,ultra thick, intersection segments={of=temp1 and green,sequence=L3}]  
    [intersection segments={of=temp1 and green, sequence={--R2}}]
;

\end{tikzpicture}

\end{document}

enter image description here

Best Answer

Related Solutions

[Tex/LaTex] Rotate a node but not its content: the case of the ellipse decoration

[Tex/LaTex] TikZ: Drawing an arc from an intersection to an intersection

Related Question