arrowstikz-pgf
I've been Googling for ages and can't find this answer. The only thing that even gets close to asking what I'm wanting is this question but I don't quite get the answer. How in the world does \pgfarrowsdeclarecombine work?
Anyway, I want to make an arrow in a TikZ picture that looks like +---->. Like a normal arrow, but with a plus sign at the beginning.
There are a number of ways you can do this in TikZ. Here are three examples:
\begin{tikzpicture}
\node[anchor=east] at (0,0) (text) {This is some text.};
\node[anchor=west] at (3,1) (description) {Here is the description.};
\draw (description) edge[out=180,in=0,->] (text);
\end{tikzpicture}
\begin{tikzpicture}
\node[anchor=east] at (0,0) (text) {This is some text.};
\node[anchor=west] at (3,1) (description) {Here is the description.};
\draw[->] (description) -| (text);
\end{tikzpicture}
\begin{tikzpicture}
\node[anchor=east] at (0,0) (text) {This is some text.};
\node[anchor=west] at (3,1) (description) {Here is the description.};
\draw[->] (description) .. controls ([xshift=-4cm] description) and ([xshift=4cm] text) .. (text);
\end{tikzpicture}
which produces
Adapting my answer at LaTeX package to help diagram translation of one language to another, I provide a first cut.
The syntax is
\notate[B<race>]{<referenced math>}{<dropline length in \baselineskips>}{<math notation>}
The <math notation> can be given as a \parbox, if longer text is needed (shown in first example below).
The MWE:
\documentclass{article}
\usepackage[usestackEOL]{stackengine}
\usepackage{scalerel}
\usepackage{graphicx,amsmath}
\parskip \baselineskip
\def\myupbracefill#1{\rotatebox{90}{\stretchto{\{}{#1}}}
\def\rlwd{.5pt}
\newcommand\notate[4][B]{%
\if B#1\else\def\myupbracefill##1{}\fi%
\def\useanchorwidth{T}%
\setbox0=\hbox{$\displaystyle#2$}%
\def\stackalignment{c}\stackunder[-6pt]{%
\def\stackalignment{c}\stackunder[-1.5pt]{%
\stackunder[2pt]{\strut $\displaystyle#2$}{\myupbracefill{\wd0}}}{%
\rule{\rlwd}{#3\baselineskip}}}{%
\strut\kern9pt$\rightarrow$\smash{\rlap{$~\displaystyle#4$}}}%
}
\begin{document}
\begin{equation*}
\implies \frac{d}{dt} \displaystyle\int_{-\infty}^{+\infty} |\psi (x, t)|^2 \, dx
= \frac{i\hbar}{2m} \left[ \psi^* \frac{\partial}{\partial x} \psi - \left( \frac{\partial}{\partial x} \psi^* \right) \psi \right] \bigg|_{-\infty}^{+\infty}
\notate[X]{{}={}}{2}{\parbox[t]{1in}{Here, $\psi$ and $\psi^*$ are the
blah-blah variables.}} 0
\hspace{5ex}
\end{equation*}
\bigskip
\[
y = \psi^*
\notate{\frac{d\psi}{dt}}{4}{
\frac{d\psi}{dt} = \frac{ih}{2m}\frac{\partial^2}{\partial t^2}\psi
- \frac{i}{h} V\psi
}
+ \notate{\frac{d\psi^*}{dt}}{1}{
\frac{d\psi*}{dt} = -\frac{ih}{2m}\frac{\partial^2}{\partial t^2}\psi^*
+ \frac{i}{h} V\psi
}
\psi
\hspace{14ex}
\]
\end{document}
Note that this solution avoids the overlapping lines shown by the OP for a multi-part comment.
Since the math notation is \rlapped, the overall effect can be shifted left as needed by adding an \hspace{<length>} to the end of the equation.
Because the math notation is \smashed, some extra skip may be needed for multi-line notations.
Best Answer
Here is a very simple solution that scales with line width: