I'm aware of the possibility given by pgfplots to have two y-axes, one on the left and one on the right with two different scales. But I'd like to explore other ways to have multiple axes, e.g. putting a separate axis besides a given plot.

I've studied the groupplot environment and tried to put a second y-axis on the left of a plot.

Here's my MWE:

```
\documentclass{article}
\usepackage{lmodern}
\usepackage{pgfplots}
\pgfplotsset{compat=1.3}
\usepgfplotslibrary{groupplots}
\begin{document}
\pgfkeys{
/pgf/number format/.cd,
set decimal separator={,{\!}},
set thousands separator={}
}
\pgfplotsset{
every axis/.append style = {
line width = 1pt,
tick style = {line width=1pt}
}
}
\begin{tikzpicture}
\begin{groupplot}[
group style = {
group size=3 by 1,
horizontal sep=1.3cm, vertical sep=0pt,
xticklabels at=edge bottom
},
height=5cm, width=9cm,
no markers
]
% this is the leftmost y axis
\nextgroupplot[
width=2cm,
hide x axis,
axis y line*=left,
ymin=0, ymax=10,
ytick = {0,1,...,10},
ylabel={\color{red}$y_2$}
]
% this is the red curve
\nextgroupplot[
xmin=0, xmax=10,
xlabel={$x$},
ymin=0, ymax=100,
ytick = {0,20,...,100},
ylabel={\color{blue}$y_1$}
]
\addplot[very thick, blue, domain=0:10] {x^2};
% this is the blue curve
\nextgroupplot[
xshift=-8.715cm,% see above: horizontal sep=1.3cm
xmin=0, xmax=10,
ymin=0, ymax=10,
hide x axis,
hide y axis,
]
\addplot[very thick, red, domain=0:10] {0.05*x^2};
\end{groupplot}
\end{tikzpicture}
\end{document}
```

that produces the following result:

Q1: Why the yticks for the red curve (label $y_2$) are wrong?

Q2: is there a more neat way to shift left the last plot (red curve), instead of setting "xshift=-8.715cm" after a trial-and-error process?

## Best Answer

As discussed in my comments, here is my proposed solution without groupplots.

To summarize the comments, I replicate them here:

Q1: It really did not find complete axis limits (because that particular axis has no data), so it uses [0,1] x [0,1] as fallback (which should be indicated by some warning in your log file). Provide some (arbitrary, but non-empty) x range will fix the problem:

`xmin=0,xmax=1`

Q2: you can provide

`\begin{axis}[options] ... \end{axis}`

several times in one picture - each time with different`options`

and with different plots. As long as the options for dimensions (like`width`

), positioning (`at`

,`xshift`

, etc), and alignment (`anchor`

) are the same, they will be drawn on top of each other.My proposed solution is to draw two axes completely on top of each other, and only the second y axis with an

`xshift`

.Here is the combined approach for your MWE: