Graphics, Figures & Tables ⇒ Approximation Lines on Graph

[tpa77l3]

Hey guys,

I'm struggling with finding out how to draw intercept lines on a graph to find approximate values on a graph like the red and blue lines below.

Screenshot 2020-03-01 at 06.11.23.png (156.59 KiB) Viewed 4058 times

I'm trying to add lines from the y-axis at 100 down to approximately 11 on the x-axis.

Code: Select all

\documentclass[border=5pt]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=1.16}

\begin{document}
\begin{tikzpicture}
\begin{axis}[
xlabel={Metres [m]},
xtick align=outside,
xmin=0,
xmax=20,
ylabel={Cost [£]},
ytick align=outside,
ymin=0,
ymax=180,
axis lines=left,
ymajorgrids=true,
xmajorgrids=true,
grid style=dashed,
]
\addplot
coordinates {(1,9) (2,18) (4,36) (5,45) (10,90) (20,180)}
;
\end{axis}
\end{tikzpicture}
\end{document}

Thanks in advance!

[Bartman]

pgf/TikZ offers the intersections library to solve this task:

Code: Select all

\documentclass[border=5pt]{standalone}
\usepackage{pgfplots}% loads the package tikz
\pgfplotsset{compat=1.16}

\usetikzlibrary{intersections}

\begin{document}
\begin{tikzpicture}
\begin{axis}[
    xlabel={Metres [m]},
%    xtick align=outside,
    xmin=0,
    xmax=20,
    ylabel={Cost [£]},
%    ytick align=outside,
    ymin=0,
    ymax=180,
    axis lines=left,
    grid=major,% <- changed
    grid style=dashed,
]
\addplot+ [name path=graph, black, mark options={black}]
    coordinates {(1,9) (2,18) (4,36) (5,45) (10,90) (20,180)}
;
\path [name path=y100] 
    (0,100) -- (0,100-|current axis.east)
;
\draw [
    -Stealth,
    red, 
    dashed,
    name intersections={of=y100 and graph, by=is}
] 
    (is|-current axis.origin) edge (is)
    (is) edge (0,100)
;
\end{axis}
\end{tikzpicture}
\end{document}

[tpa77l3]

To the rescue again! Much appreciated.

Even now knowing that this is called intersections, I'm still none the wiser having looked it up in the manual! It's just what I'm after apart from the arrows should be on the other side of the dashed lines. Try as I may I can't even figure out to do this! If you could point (pun intended) me in the right direction that, again, would be greatly appreciated.

Could you recommend any decent learning sources for bettering my Tex knowledge? I'm working my way through several books. I guess most of its learnt by doing, but it would be good to know if there is any online courses that I may have missed.

[Bartman]

It's just what I'm after apart from the arrows should be on the other side of the dashed lines. Try as I may I can't even figure out to do this!

I accidentally used the command \draw and should have used \path instead. In this case it may work, but unfortunately not for your purpose. You have to replace \draw with \path and swap the node names so that you get the desired position of the arrows.

Could you recommend any decent learning sources for bettering my Tex knowledge? I'm working my way through several books. I guess most of its learnt by doing, but it would be good to know if there is any online courses that I may have missed.

I am not familiar with LaTeX introductions in English. In my experience, the package documentation and the posts in the forums about pgfplots and pgf/TikZ are sufficient to familiarize yourself with the matter. Of course you need a lot of time to practice.

[tpa77l3]

Thanks for the reply. I'm still having trouble figuring out what needs swapping. Sorry for being dense!

[Bartman]

The documentation describes the content of the parentheses as coordinates. A lot can be inserted at this point: the x and y value of a coordinate, the name of a node, the name of a coordinate determined with the \coordinate command or the name of a calculated intersection point, etc. Combinations of these, as shown in the example, are also permitted. In your case, the round pairs of parentheses must be swapped, which are before and after the word edge.

A simplified example of my code:

Code: Select all

\draw [...]% replace the command for your purpose, but keep the options
    (a) edge (b)% swap
    (b) edge (c)% the same as before
;

[tpa77l3]

Got it- many thanks.