pgfplot - Removing whitespace under x axis produced by 'units' library

Question

I was wondering if anyone has had the same results which I'm having when using the library units from pgfplots. When the options change x base=true, x SI prefix=milli are used, pgfplots seems to put an 'extra' whitespace under the x axis. Here is a MWE:

\documentclass[class=IEEEtran]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\usepgfplotslibrary{units}
\usetikzlibrary[backgrounds]
\definecolor{graphicbackground}{rgb}{0.96,0.96,0.8}

\begin{document}
\begin{tikzpicture}[tight background ,show background rectangle,
 background rectangle/.style={fill=graphicbackground}]%
\begin{axis}[%
scale only axis,
xlabel={$t$ (ms)},
ylabel={$f(t) = t^2 - t +4$},
xmin=1.8E-3, xmax=8.5E-3,
ymin=-8.5, ymax= -2.5,
axis x line=bottom,
axis y line=left,
axis line style = {-latex},
change x base=true, x SI prefix=milli,
every axis x label/.style={at={(ticklabel* cs:1.0)},anchor=south east},
]%
\addplot[color=red,mark=x] coordinates {
    (2E-3,-2.8559703)
    (3E-3,-3.5301677)
    (4E-3,-4.3050655)
    (5E-3,-5.1413136)
    (6E-3,-6.0322865)
    (7E-3,-6.9675052)
    (8E-3,-7.9377747)
};
\end{axis}%
\end{tikzpicture}
\end{document}

Which produces the following:

As opposed when the x-axis is in the range from '0' to '10', in which case the 'units' library is not used

\documentclass[class=IEEEtran]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\usepgfplotslibrary{units}
\usetikzlibrary[backgrounds]
\definecolor{graphicbackground}{rgb}{0.96,0.96,0.8}

\begin{document}
\begin{tikzpicture}[tight background ,show background rectangle,
 background rectangle/.style={fill=graphicbackground}]%
\begin{axis}[%
scale only axis,
xlabel={$t$ (ms)},
ylabel={$f(t) = t^2 - t +4$},
%xmin=1.8E-3, xmax=8.5E-3,
xmin=1.8, xmax=8.5, % <- new x values
ymin=-8.5, ymax= -2.5,
axis x line=bottom,
axis y line=left,
axis line style = {-latex},
%change x base=true, x SI prefix=milli,
every axis x label/.style={at={(ticklabel* cs:1.0)},anchor=south east},
]%
\addplot[color=red,mark=x] coordinates {
    (2,-2.8559703) %<- x values changed
    (3,-3.5301677)
    (4,-4.3050655)
    (5,-5.1413136)
    (6,-6.0322865)
    (7,-6.9675052)
    (8,-7.9377747)
};
\end{axis}%
\end{tikzpicture}
\end{document}

which outputs the following:

Notice in the last image there is less whitespace under the x-axis. Does anyone knows what might be causing this? and how can I get rid of this extra whitespace? I would appreciate your comments.

Answer 1

In your first case, without the scaling done by the units library, the ticks would be scaled, and there would be a multiplier present below the x-axis, i.e. the $\cdot 10^{-3}$ in this screenshot.

With the scaling done by the units library, that node is empty, but it's still there, and thus it affects the bounding box. Add every x tick scale label/.append style={draw,minimum size=1cm} to see that it's there.

Hence, you need to move that node, e.g. by adding at={(rel axis cs:0.5,0.5)} to the style. Or you can add overlay to the style, which means TikZ ignores it when calculating the bounding box.

\documentclass[tikz]{standalone}
\usepackage{pgfplots}
\pgfplotsset{compat=newest}
\usepgfplotslibrary{units}
\usetikzlibrary[backgrounds]
\definecolor{graphicbackground}{rgb}{0.96,0.96,0.8}

\begin{document}
\begin{tikzpicture}[tight background ,show background rectangle,
 background rectangle/.style={fill=graphicbackground}]%
\begin{axis}[%
scale only axis,
xlabel={$t$ (ms)},
ylabel={$f(t) = t^2 - t +4$},
xmin=1.8E-3, xmax=8.5E-3,
ymin=-8.5, ymax= -2.5,
axis x line=bottom,
axis y line=left,
axis line style = {-latex},
change x base=true, x SI prefix=milli,
every axis x label/.style={at={(ticklabel* cs:1.0)},anchor=south east},
every x tick scale label/.append style={at={(rel axis cs:0.5,0.5)}} %%% <-- 
]%
\addplot[color=red,mark=x] coordinates {
    (2E-3,-2.8559703)
    (3E-3,-3.5301677)
    (4E-3,-4.3050655)
    (5E-3,-5.1413136)
    (6E-3,-6.0322865)
    (7E-3,-6.9675052)
    (8E-3,-7.9377747)
};
\end{axis}%
\end{tikzpicture}
\end{document}