# Prevent pgfplots from scaling axis

I want three plots, with a constant Y-value. I need these to be visually comparable, so the Y-axis scale needs to be the same. The three values I'm plotting are: y= {0.3,0.6,0.9}, and my Y-axis should show between 0 and 1.

Here's what I have:

\documentclass{standalone}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
width=7cm,
height=5cm,
align=center,
xticklabels={},
ymax = 1.1,xmax=3.5,
axis lines*=center,
ytick={0.5,1},
xtick={1,2,3},
xlabel shift= 0.4cm]
\addplot+[blue,thick,mark=none] coordinates {(0, 0.9) (3, 0.9)};;
\end{axis}
\end{tikzpicture}
\end{document}

This produces:

And, when I plot y=0.3, I get this:

The Y-axis looks like it's been scaled: the line appears in the same place in the image, but the axis markings have shifted.

How can I prevent pgfplots from doing this?

The axis isn't being scaled - it's being shifted.

The second image gives the impression of an exponential axis scale, because the first mark (0.5) is closer to the X-axis than it is to the second mark (1) - but this is not the case, which can be demonstrated by adding in more tick marks:

To prevent this from occuring, set the ymin to 0:

The MWE to produce the second of these corrected plots:

\documentclass{standalone}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}[
width=7cm,
height=5cm,
align=center,
xticklabels={},
ymax = 1.1,xmax=3.5,
ymin = 0, xmin = 0,
axis lines*=center,
ytick={0.25, 0.5,0.75, 1},
xtick={1,2,3},
xlabel shift= 0.4cm]
\addplot+[blue,thick,mark=none] coordinates {(0, 0.9) (3, 0.9)};;
\end{axis}
\end{tikzpicture}
\end{document}