pgfplotstikz-pgf
I'm looking to design a question where students sketch a derivative based on a drawn function without the y-axis.
The y-axis is not important as we're focusing on the x-values of turning points and inflection points (in fact I don't need it at all).
I haven't managed to find a way to graph a function with only the x-axis (without the ticks or numbers is even better) with tikz or pgfplots.
Does anyone know how to do this?
Thanks in Advance!
If you define your function using \def\<function macro>(#1){\pgfmathparse{...#1...}, you can use the function in all the applications you mentioned by saying \<function macro>(x) or \<function macro>(x-1), or \<function macro>(2.4). This will work both when using Gnuplot and when using straight PGFplots:
\documentclass[border=5pt]{standalone}
\usepackage{pgfplots}
\def\FunctionF(#1){(#1)^3}%
\newcommand*{\SpecialXValue}{1.5}%
\begin{document}
\begin{tikzpicture}
\begin{axis}[
domain=-3:3, samples=50,
enlargelimits=false,
ymin=-5, ymax=5,
every axis plot post/.style= ultra thick]
\addplot [red] gnuplot {\FunctionF(x)};%
\addplot [blue] gnuplot {\FunctionF(x+1)};%
\addplot [cyan] {(x<1)*(3-3*x)+!(x<1)*-\FunctionF(x-1.5))};
\pgfmathsetmacro{\specialY}{\FunctionF(-1.5)}
\fill (axis cs:-1.5,\specialY) circle [radius=2pt] node [right] {(2,\specialY)};
\end{axis}
\end{tikzpicture}
\end{document}
Old Answer:
You can redefine the x function to return a constant value (in your case \SpecialXValue) before calculating the y value. The easiest way to do that is to issue
\tikzset{declare function={x=\SpecialXValue;}}
just before you calculate the y value.
You can still add new functions after this, the definition of x as a fixed value doesn't persist.
\documentclass[border=5pt]{standalone}
\usepackage{pgfplots}
\newcommand*{\XAxisMin}{-1.0}
\newcommand*{\XAxisMax}{3.0}
\newcommand*{\YAxisMin}{-2.0}
\newcommand*{\YAxisMax}{10}
\newcommand*{\DomainMinF}{\XAxisMin}
\newcommand*{\DomainMaxF}{2.2}
\pgfkeys{/pgfplots/Axis Style/.style={
xmin=\XAxisMin, xmax=\XAxisMax,
ymin=\YAxisMin, ymax=\YAxisMax,
domain=\DomainMinF:\DomainMaxF,
width=6.5cm
}}
% Gnuplot options here have no effect if not using GnuPlot
\pgfkeys{/pgfplots/Plot Style/.style={
translate gnuplot=true,% can use ‘^’ instead of ‘**’
id=foo,
mark=none,%
domain=\DomainMinF:\DomainMaxF,%
samples=50,%
ultra thick,
}}
\newcommand*{\LabelPoint}[2]{\fill (axis cs:#1,#2) circle [radius=3pt] node [right] {\footnotesize$(#1,#2)$};}%
\newcommand*{\FunctionFGnuplot}{(x)^3}%
\newcommand*{\SpecialXValue}{1.1}%
\begin{document}
\begin{tikzpicture}
\begin{axis}[Axis Style]
\addplot [Plot Style, red] gnuplot {\FunctionFGnuplot};%
\tikzset{declare function={x=\SpecialXValue;}}
\pgfmathsetmacro{\YatSpecialX}{\FunctionFGnuplot}%
\LabelPoint{\SpecialXValue}{\YatSpecialX}
\end{axis}
\end{tikzpicture}
\end{document}
The scaled x ticks key only acts after the tick positions have already been determined, so they're not the way to go here. Instead, you should transform your coordinate system using x coord trafo/.code and y coord trafo/.code.
Note that for values as large as yours, you'll need to switch on the fpu library in the .code. When using xmin, xmax, etc., the fpu library needs to be deactivated after the calculation, but when plotting mathematical expressions, the library may not be deactivated. This is a bit cumbersome: You'll have to \pgflibraryfpuifactive to decide whether we're in a context where the libary should be switched on and off, or left as it is:
\documentclass{standalone}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
xmin=0, xmax=70000,
ymin=0, ymax=5000,
domain=0:70000,
x coord trafo/.code={
\pgflibraryfpuifactive{
\pgfmathparse{(#1)/(19099)}
}{
\pgfkeys{/pgf/fpu=true}
\pgfmathparse{(#1)/(19099)}
\pgfkeys{/pgf/fpu=false}
}
},
y coord trafo/.code={
\pgflibraryfpuifactive{
\pgfmathparse{(#1)/(2000)}
}{
\pgfkeys{/pgf/fpu=true}
\pgfmathparse{(#1)/(2000)}
\pgfkeys{/pgf/fpu=false}
}
},
]
\addplot {2000+0.001*(x/60)^2};
\end{axis}
\end{tikzpicture}
\end{document}
Best Answer
What you're looking for is
axis y line=none: