Unwanted stretch and translation of an additional axis

Question

Consider the following code:

\documentclass{standalone}
\usepackage{tikz,pgfplots,siunitx}
\pgfplotsset{compat=newest}
\begin{document} 
\begin{tikzpicture}

%% 1st and 2nd waveforms
\begin{axis}[
    scale only axis,
    axis y line*=left,
    axis x line*=bottom,
    xlabel={Time [\si{s}]},
    ylabel={Current [\si{\A}]},
    grid=major
]
\addplot [blue, line width = 1, smooth] 
table[x=t,y=ch1,col sep=comma]{myDataFile.csv};
\addplot [green, line width = 1, smooth] 
table[x=t,y=ch2,col sep=comma]{myDataFile.csv};
\end{axis}
%% Here starts the 3rd waveform
\begin{axis}[
    scale only axis,
    axis y line*=right,
    axis x line*=bottom,
    xlabel={Time [\si{s}]},
    ylabel={Voltage (Normalised)}
    %grid=major
]
\addplot [red, line width = 1, smooth]
table[x=t,y=ch3,col sep=comma]{myDataFile.csv};
\end{axis}
   
\end{tikzpicture}
\end{document}

The myDataFile.csv file is the following:

%myDataFile.csv%
t,ch1,ch2,ch3
-4,-2,0.0625,0.756
-3,-1.5,0.125,-0.141
-2,-1,0.25,-0.909
-1,-0.5,0.5,-0.841
0,0,1,0
1,0.5,2,0.841
2,1,4,0.909
3,1.5,8,0.141
4,2,16,-0.758

The thing is that the 3rd waveform is translated (up about 6 units) and also stretched, so that it extends the whole vertical span determined by the minimum value of ch1 or ch2, the smaller of the two, and the maximum value of ch1 or ch2, the larger of the two.

In the example above, ch3 (red) is a sine function plotted from t=-4 to t=+4, so it is bounded from -1 to 1 in the vertical direction. As you can see, it is not plotted correctly. (ch1 is y=-t/2, and ch2 is 2^t)

I am following the code in this answer, about plotting two functions on one "canvas".

From what I see in the inline picture of this linked answer, the same effect applies there too.

How can I avoid this and plot ch3 as it is?

To elucidate my point, I want something that looks like the following (plotted in Desmos):

Notice how the sine wave spans in the vertical direction compared with the 2^t curve. It is really from -1 to +1 and its "x-axis" coincides with that of the other two curves. But, it describes a different physical quantity and therefore "deserves" a different label and ticks, to be plotted on the right edge.

Moreover, how can I instruct pgfplots to stretch it in the vertical direction by, say, 2 times each value entry? I found a question about scaling a data set, but I don't know how to do it for data imported from a csv file as in the case here.

Answer 1

You need to set the y-limits to the same values for both axes, so add ymin=-4, ymax=17. There are several ways to do that, you can add it to both axes separately, you can do \pgfplotsset{common axis settings/.style={ymin=-4, ymax=17}} and then add common axis settings to both axis options, or simply add \pgfplotsset{ymin=-4, ymax=17} between \begin{tikzpicture} and the first \begin{axis}.

I'd probably remove smooth, which will use bezier curves between sample points instead of straight lines, and the x-ticks and label from the second axis (which currently are duplicated).

As mentioned in the comment, you can use the method for scaling plots described by Jake in the answer you link to, I demonstrated that in code below as well.

\documentclass[border=5mm]{standalone}
\usepackage{pgfplots,siunitx}
\pgfplotsset{compat=newest}
\begin{document} 
\begin{tikzpicture}
\pgfplotsset{
  ymin=-4,
  ymax=17,
  every axis plot/.append style={
    line width = 1,
    % if you want to add markers as in the Desmos plot, uncomment this
%      mark=*,
%      mark size=2
     }
  }
%% 1st and 2nd waveforms
\begin{axis}[
    scale only axis,
    axis y line*=left,
    axis x line*=bottom,
    xlabel={Time [\si{s}]},
    ylabel={Current [\si{\A}]},
    grid=major
]
\addplot [blue] table[x=t,y=ch1,col sep=comma] {myDataFile.csv};
\addplot [green] table[x=t,y=ch2,col sep=comma] {myDataFile.csv};
\end{axis}

%% Here starts the 3rd waveform
\begin{axis}[
    scale only axis,
    axis y line*=right,
    axis x line*=bottom,
    xtick=\empty,
%    xlabel={Time [\si{s}]},
    ylabel={Voltage (Normalised)},
    ylabel style={red}
]
\addplot [red] table[x=t,y=ch3,col sep=comma] {myDataFile.csv};
\addplot [red, densely dashed] table[x=t,y expr=\thisrow{ch3}*2,col sep=comma] {myDataFile.csv};
\end{axis}
   
\end{tikzpicture}
\end{document}