baseline of custom marks in pgfplots

Question

I'm trying to plot some intervals with pgfplots and wanted to use a custom mark for upper and lower bound. I planned doing this by using a "V" for the upper bound and a "^" for the lower bound (ideally not the letters but symbols that look like the upper and lower half of an X). The idea here is that both symbols will merge into an X for empty intervals.

I guess I could get symbols like those with text mark={v} but the alignment would be off, as pgfplots would center the mark instead of using the tip of the "v" as its baseline.

Any idea how I could get the alignment right?

Answer 1

PGFPlots can use any plot mark which is defined in pgf. You may want to study pgflibraryplotmarks.code.tex (which is shipped with pgf) and define a mark accordingly.

The alignment is simple: at the time when plot marks are drawn, the origin (0pt,0pt) is the marks position. Simply make sure that your V has y>0pt and ^ has y < 0pt, that's all.

Answer 2

I found a solution after looking in the pgfplots source code. With the following definition I got the plot marks I wanted

\pgfdeclareplotmark{v}{%
  \pgfpathmoveto{\pgfpoint{-\pgfplotmarksize}{\pgfplotmarksize}}
  \pgfpathlineto{\pgfpoint{0pt}{0pt}}
  \pgfpathlineto{\pgfpoint{\pgfplotmarksize}{\pgfplotmarksize}}
  \pgfusepathqstroke
}
\pgfdeclareplotmark{^}{%
  \pgfpathmoveto{\pgfpoint{-\pgfplotmarksize}{-\pgfplotmarksize}}
  \pgfpathlineto{\pgfpoint{0pt}{0pt}}
  \pgfpathlineto{\pgfpoint{\pgfplotmarksize}{-\pgfplotmarksize}}
  \pgfusepathqstroke
}

After the definition the marks can be used like any other mark, e.g.

\addplot[only marks, mark=^] coordinates { (0,0) }
\addplot[only marks, mark=v] coordinates { (0,1) }
\addplot[mark=none] coordinates { (0,0) (0,1) }