tikz - relative coordinates of a second position

I'm drawing a kind of sequence diagram but I don't want to calculate every position.

I want to use relative coordinates. With the command coodinate I set the first coordinate (0.25,14.25) as a, but I want to select a second coordinate (0.25,14.75).

Is there a way to do this?

%Actor
\draw[dashed] (0.25,15) -- (0.25,0)  node[above=15] {User};
\draw[dashed] (6,15) -- (6,0)  node[above=15] {HMI};
\draw[dashed] (12,15) -- (12,0) node[above=15] {Server1};
\draw[dashed] (18,15) -- (18,0) node[above=15] {Server2};
%
%Blocks
\begin{scriptsize}
\draw[line width=5pt] (0.25,14.25) -- (0.25,14.75)  node[midway, right] {Test} coordinate (a);
\end{scriptsize}
%
%Arrows
\begin{scope}[>=latex]
\draw[->,black] (a) -- (6,14);
\end{scope}

• Welcome. Please can you add a complete MWE? What are packages that you use? – Sebastiano Mar 5 '17 at 20:16

You can do this using the calc library (See Section 13.5 Coordinate Calculations of the pgf manual):

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calc}
\begin{document}

\begin{tikzpicture}
\coordinate (a) at (2,2);
\coordinate (b) at (0,-2);
\node[draw=red] at (a) {a};
\node[draw=red] at (b) {b};
\draw[help lines] (-1,-3) grid (3,3);
\draw ($(a) + (0,1)$) -- ($(b) + (0,1)$);
\end{tikzpicture}
\end{document}


I think you have it a bit confused. Your coordinate (a) (which is an empty node) is really (0.25,14.75), at the end of the path.

To have a "marker" for the start and end of the path you can do:

    %Blocks
\begin{scriptsize}
\draw[line width=5pt] (0.25,14.25) coordinate(b) --
(0.25,14.75)  node[midway, right] {Test} coordinate (a);
\end{scriptsize}
%
%Arrows
\begin{scope}[>=latex]
\draw[->,black] (a) -- ++(1,0) node {a};
\draw[->,black] (b) -- ++(1,0) node {b};
\end{scope}


and you'll have (partial graph):

In general, you can have node on whichever part of the path, with the pos key; if you add

        \draw[line width=5pt] (0.25,14.25) coordinate(b) --
(0.25,14.75)  node[midway, right] {Test}
coordinate (a)  coordinate[pos=0.5](c);


and

       \draw[->,black] (c) -- ++(2,0) node {c};


you have:

Notice also that the midway keys is really another way of saying pos=0.5.

First Answer: Use the package tikz-uml, it offers also support for sequence diagrams. See perso.ensta-paristech.fr/~kielbasi/tikzuml; the sequence diagrams are documented here.

Second Answer: If you want to do it yourself using the tikz commands directly, I suggest to

• use commands for constants, like \xUser and \xHmi for the horizontal positions of the time lines

• use nodes for actors and actions and name them

• define styles for the parameters of the nodes.

Then it will be easy to adjust and extend the diagram. Here is an example how you can do it.

\documentclass[border=3pt]{standalone}
\usepackage{tikz}
\tikzset
{actor/.style={font=\ttfamily},
action/.style 2 args= % first arg is length in mm, second arg is label
{minimum width=3mm,minimum height=#1mm,inner sep=0pt,
draw,fill=blue!30,anchor=north,label={right:\scriptsize#2}
},
message/.style={-stealth,font=\scriptsize\bfseries},
timeline/.style={dashed}
}
\begin{document}
\begin{tikzpicture}[x=1mm,y=1mm]
% horizontal position of actors (in mm)
\newcommand\xUser{0}
\newcommand\xHmi{20}
\newcommand\xSa {40}
\newcommand\xSb {60}
% actors
\node[actor] (user-0) at (\xUser,0) {User};
\node[actor] (hmi-0)  at (\xHmi ,0) {HMI};
\node[actor] (s1-0)   at (\xSa  ,0) {Server1};
\node[actor] (s2-0)   at (\xSb  ,0) {Server2};
% time lines
\newcommand\yTimeMax{-65}
\draw[timeline] (user-0) -- +(0,\yTimeMax); % example for relative positioning
\draw[timeline] (hmi-0)  -- +(0,\yTimeMax);
\draw[timeline] (s1-0)   -- +(0,\yTimeMax);
\draw[timeline] (s2-0)   -- +(0,\yTimeMax);
% actions (first arg is length in mm, second arg is label)
\node[action={10}{$A_1$}] (user-1) at (\xUser,-10) {};
\node[action={ 5}{$A_2$}] (hmi-1)  at (\xHmi ,-20) {};
\node[action={15}{$A_3$}] (s1-1)   at (\xSa  ,-25) {};
\node[action={10}{$A_4$}] (user-2) at (\xUser,-50) {};
% messages
\draw[message] (user-1.south east) -- node[above]{a} (hmi-1.north west);
\draw[message] (hmi-1.south east)  -- node[above]{b} (s1-1.north west);
\draw[message] (s1-1.south west)   -- node[above]{c} (user-2.north east);
\end{tikzpicture}
\end{document}