How to use fewer markers while keeping curve details?

Question

I am trying to add a few markers to a curve with lots of points, like shown below, but without resorting to the second addplot:

\documentclass{article}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}
  \addplot[color=red,no markers] coordinates {
    (2,-1.8559703)
    (2.5,-2.8559703)
    (3,-3.5301677)
    (3.5,-4.01677)
    (4,-4.3050655)
    (4.5,-4.813136)
    (5,-5.1413136)
    (5.5,-5.7322865)
    (6,-6.0322865)
    (6.5,-6.675052)
    (7,-6.9675052)
    (7.5,-7.975052)
    (8,-8.9377747)
  };
  \addplot[color=red, only marks] coordinates {
    (2,-1.8559703)
    (4,-4.3050655)
    (6,-6.0322865)
    (8,-8.9377747)
  };
\end{axis}
\end{tikzpicture}
\end{document}

Answer 1

You have several keys allowing to control which markers should be drawn:

1. You can use mark repeat=<number>; this allows to draw only each nth mark where n was the value provided as <number>.

2. mark phase=<number>; add this after using mark repeat to control the starting point for the markers.

3. There also mark indices={<index list>}; use this to specify the index list of markers that should be drawn.

A complete example showing those keys in action (the first example gives the marking schema you requested in your question):

\documentclass{article}
\usepackage{pgfplots}
\begin{document}
\begin{tikzpicture}
\begin{axis}
  \addplot+[color=red,mark repeat=4] coordinates {
    (2,-1.8559703)
    (2.5,-2.8559703)
    (3,-3.5301677)
    (3.5,-4.01677)
    (4,-4.3050655)
    (4.5,-4.813136)
    (5,-5.1413136)
    (5.5,-5.7322865)
    (6,-6.0322865)
    (6.5,-6.675052)
    (7,-6.9675052)
    (7.5,-7.975052)
    (8,-8.9377747)
  };
\end{axis}
\end{tikzpicture}

\begin{tikzpicture}
\begin{axis}
  \addplot+[color=red,mark repeat=3,mark phase=2] coordinates {
    (2,-1.8559703)
    (2.5,-2.8559703)
    (3,-3.5301677)
    (3.5,-4.01677)
    (4,-4.3050655)
    (4.5,-4.813136)
    (5,-5.1413136)
    (5.5,-5.7322865)
    (6,-6.0322865)
    (6.5,-6.675052)
    (7,-6.9675052)
    (7.5,-7.975052)
    (8,-8.9377747)
  };
\end{axis}
\end{tikzpicture}

\begin{tikzpicture}
\begin{axis}
  \addplot+[color=red,mark indices={1,5,9}] coordinates {
    (2,-1.8559703)
    (2.5,-2.8559703)
    (3,-3.5301677)
    (3.5,-4.01677)
    (4,-4.3050655)
    (4.5,-4.813136)
    (5,-5.1413136)
    (5.5,-5.7322865)
    (6,-6.0322865)
    (6.5,-6.675052)
    (7,-6.9675052)
    (7.5,-7.975052)
    (8,-8.9377747)
  };
\end{axis}
\end{tikzpicture}

\end{document}