# Fading of horizontal lines is impossible?

The path fading option in TikZ is obviously not working for horizontal lines. For plotting constant functions as well as for drawing a line between two points. Making them slightly tilted solves the problem. Is this a know error and is there some way to work around?

Here is a minimal example

\documentclass[a4paper,11pt]{article}
\usepackage{tikz,pgfplots}
\begin{document}
\begin{tikzpicture}[>=stealth]
\begin{axis}[scale=3, domain=-1.1:6.5, xmin=-1, xmax=6.5,ymax=2, axis lines=none]

\draw[red,dashed,path fading=east] (axis cs:-1,0.9) -- (axis cs:6.5,0.9); %not working
\draw[red,dashed,path fading=east] (axis cs:-1,0.9) -- (axis cs:6.5,1);

\end{axis}

\end{tikzpicture}
\end{document}

• Welcome to TeX.SX. Usually, we don't put a greeting or a "thank you" in our posts. While this might seem strange at first, it is not a sign of lack of politeness, but rather part of our trying to keep everything very concise. Accepting and upvoting answers is the preferred way here to say "thank you" to users who helped you. – Claudio Fiandrino Mar 11 '13 at 18:35
• My suspicion is that the bounding box of a horizontal path has zero height (this is backed up by the error message of PGF trying to divide 1 by 0.0). Tilting it fixes that. Perhaps something that added a bit of height to the bounding box would fix this (but I don't know how to do that). – Andrew Stacey Mar 11 '13 at 18:37
• @AndrewStacey: You could just append ++(0,1pt) to the \addplot command and to the path. – Jake Mar 11 '13 at 18:48
• @Jake That would be a hack! Better would be to fix the fading so that it used a minimal sized bounding box. Or use a key that put in a box of the right size via an append after command type argument. – Andrew Stacey Mar 11 '13 at 20:19

Well, it's possible but you have to cooperate with TikZ. What you are doing is essentially trying to fit the fading which is defined on a nonzero area to a zero area with (nonzero width). The fadings library offers another key fit fading with which you can turn off the fading resizing accordingly to the path but this will fade regardless of your path length. Or you can simply use what Jake has suggested, namely extending the bounding box a little to get a nonzero area without actually drawing something. Or you need to define your own custom fading which should be tricky in this case. Anyway here is an example with the first two options:

\documentclass[a4paper,11pt]{article}
\usepackage{pgfplots}
\pgfplotsset{compat=1.7}

\begin{document}
\begin{tikzpicture}[>=stealth]
\begin{axis}[domain=-1.1:6.5, xmin=-1, xmax=6.5,ymax=2, axis lines=none]

\draw[red,dashed,path fading=east] (axis cs:-1,0.9) -- (axis cs:6.5,0.9) ++(0,1pt); %now working
\draw[red,dashed,path fading=east] (axis cs:-1,0.9) -- (axis cs:6.5,1);

\end{axis}

\end{tikzpicture}
\end{document}


Note that you are not actually fading a path, you are seeing the underlying fading through the path hence zero height means you can't see anything if you try to fit. That's what TikZ is complaining about.

• On my system, when I compile this answer without any modifications, I do not see any fading on either of the four lines, they are all solid. Is this answer still valid? Or am I missing something on my system that causes the fading to break? – lanoxx Nov 7 '16 at 14:23
• @lanoxx Try a different viewer – percusse Nov 7 '16 at 15:02
• ouch, I was using evince on Ubuntu (its the standard Gnome PDF viewer), using Okular from KDE solves the issue. Too bad, I really like evince. – lanoxx Nov 7 '16 at 15:10

There are several ways to extend the path in such a way that its bounding box gets a positive surface.

\draw[red,dashed,path fading=east] (axis cs:-1,0.9) -- (axis cs:6.5,0.9)
++(0,1pt); % extend the path with a degenerate subpath (see Percusse)

\draw[red,dashed,path fading=east] (axis cs:-1,0.9) -- (axis cs:6.5,0.9)
++(0,1pt) -- ++(0,0); % extend the path with a zero length subpath

\draw[red,dashed,path fading=east] (axis cs:-1,0.9) -- (axis cs:6.5,0.9)
-- ++(0,1pt); % extend the subpath


The best solution is to use a degenerate subpath (only one coordinate, Percusse), although it seems a bit of a hack. But the other two only work with the default fading.

For the other two solutions, the path extension must be on the faded edge of the bounding box, (here the right side/east side) so that you cannot see them. With extremely thick lines, you will however see glitches, but it is probably better to use rectangles in that case.

Another solution is to draw a faded white rectangle over the line, and a white line over the rectangle's faded border to remove glitches. If this is possible, then this is even a better solution, because it works for evince as well.

• I understand from your profile description that you don't like the action taken on your previous post. Note that the action was not done by moderators, but by the community. Also, as mentioned before, this is not a forum... so even though you want to comment on things "for the greater good", that can't be done really by new users. You have to gain trust (through reputation, at least 50) in the community before your contributions can be added through comments. That's just the way it is... – Werner Jul 7 '15 at 23:45
• ...how is your code snippet different from percusse's answer? Please explain, since I don't know TikZ that well... and it looks like you're saying basically the same. – Werner Jul 7 '15 at 23:46
• @Werner. The -- before the ++. -- extends the subpath, no -- starts a new one. ++ means realtive to the previous point. If they would not have been moderators in my definition, then -3 instead of 0 in front would have been the effect, not this. – brac37 Jul 7 '15 at 23:50
• You shouldn't add -- . The use of -- is not relevant here. Because the fading works like a fill. It should not be drawn because it does not belong to the path. That's why I omitted it and that's why I voted to delete because this is going in the wrong direction with the identical message that I have in my answer. This needs to be a comment if you prefer -- – percusse Jul 8 '15 at 0:59
• The bounding box is also updated without --. That's why my answer works. Hence I don't know what you are trying to convey here. – percusse Jul 8 '15 at 1:04