I have posted posted an answer before which realizes a vertical timeline for another question, but I think this may fit your needs. You can make multi line entries for each timeline entry and multiple entries for each year. Note that it may need some tweaking here and there to fit your needs.
Also note that I'm posting this answer as a community wiki entry because I have posted this same answer before. If you want to give credit for it, please do it at the original question: High level macros and environments based on Tikz.
\documentclass[10pt]{article}
\usepackage[paperwidth=210mm,%
paperheight=297mm,%
tmargin=7.5mm,%
rmargin=7.5mm,%
bmargin=7.5mm,%
lmargin=7.5mm,
vscale=1,%
hscale=1]{geometry}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{tikz}
\usetikzlibrary{arrows, calc, decorations.markings, positioning}
\pagestyle{empty}
\makeatletter
\newenvironment{timeline}[6]{%
% #1 is startyear
% #2 is tlendyear
% #3 is yearcolumnwidth
% #4 is rulecolumnwidth
% #5 is entrycolumnwidth
% #6 is timelineheight
\newcommand{\startyear}{#1}
\newcommand{\tlendyear}{#2}
\newcommand{\yearcolumnwidth}{#3}
\newcommand{\rulecolumnwidth}{#4}
\newcommand{\entrycolumnwidth}{#5}
\newcommand{\timelineheight}{#6}
\newcommand{\templength}{}
\newcommand{\entrycounter}{0}
% https://tex.stackexchange.com/questions/85528/checking-whether-or-not-a-node-has-been-previously-defined
% https://tex.stackexchange.com/questions/37709/how-can-i-know-if-a-node-is-already-defined
\long\def\ifnodedefined##1##2##3{%
\@ifundefined{pgf@sh@ns@##1}{##3}{##2}%
}
\newcommand{\ifnodeundefined}[2]{%
\ifnodedefined{##1}{}{##2}
}
\newcommand{\drawtimeline}{%
\draw[timelinerule] (\yearcolumnwidth+5pt, 0pt) -- (\yearcolumnwidth+5pt, -\timelineheight);
\draw (\yearcolumnwidth+0pt, -10pt) -- (\yearcolumnwidth+10pt, -10pt);
\draw (\yearcolumnwidth+0pt, -\timelineheight+15pt) -- (\yearcolumnwidth+10pt, -\timelineheight+15pt);
\pgfmathsetlengthmacro{\templength}{neg(add(multiply(subtract(\startyear, \startyear), divide(subtract(\timelineheight, 25), subtract(\tlendyear, \startyear))), 10))}
\node[year] (year-\startyear) at (\yearcolumnwidth, \templength) {\startyear};
\pgfmathsetlengthmacro{\templength}{neg(add(multiply(subtract(\tlendyear, \startyear), divide(subtract(\timelineheight, 25), subtract(\tlendyear, \startyear))), 10))}
\node[year] (year-\tlendyear) at (\yearcolumnwidth, \templength) {\tlendyear};
}
\newcommand{\entry}[2]{%
% #1 is the year
% #2 is the entry text
\pgfmathtruncatemacro{\lastentrycount}{\entrycounter}
\pgfmathtruncatemacro{\entrycounter}{\entrycounter + 1}
\ifdim \lastentrycount pt > 0 pt%
\node[entry] (entry-\entrycounter) [below of=entry-\lastentrycount] {##2};
\else%
\pgfmathsetlengthmacro{\templength}{neg(add(multiply(subtract(\startyear, \startyear), divide(subtract(\timelineheight, 25), subtract(\tlendyear, \startyear))), 10))}
\node[entry] (entry-\entrycounter) at (\yearcolumnwidth+\rulecolumnwidth+10pt, \templength) {##2};
\fi
\ifnodeundefined{year-##1}{%
\pgfmathsetlengthmacro{\templength}{neg(add(multiply(subtract(##1, \startyear), divide(subtract(\timelineheight, 25), subtract(\tlendyear, \startyear))), 10))}
\draw (\yearcolumnwidth+2.5pt, \templength) -- (\yearcolumnwidth+7.5pt, \templength);
\node[year] (year-##1) at (\yearcolumnwidth, \templength) {##1};
}
\draw ($(year-##1.east)+(2.5pt, 0pt)$) -- ($(year-##1.east)+(7.5pt, 0pt)$) -- ($(entry-\entrycounter.west)-(5pt,0)$) -- (entry-\entrycounter.west);
}
\newcommand{\plainentry}[2]{% plainentry won't print date in the timeline
% #1 is the year
% #2 is the entry text
\pgfmathtruncatemacro{\lastentrycount}{\entrycounter}
\pgfmathtruncatemacro{\entrycounter}{\entrycounter + 1}
\ifdim \lastentrycount pt > 0 pt%
\node[entry] (entry-\entrycounter) [below of=entry-\lastentrycount] {##2};
\else%
\pgfmathsetlengthmacro{\templength}{neg(add(multiply(subtract(\startyear, \startyear), divide(subtract(\timelineheight, 25), subtract(\tlendyear, \startyear))), 10))}
\node[entry] (entry-\entrycounter) at (\yearcolumnwidth+\rulecolumnwidth+10pt, \templength) {##2};
\fi
\ifnodeundefined{invisible-year-##1}{%
\pgfmathsetlengthmacro{\templength}{neg(add(multiply(subtract(##1, \startyear), divide(subtract(\timelineheight, 25), subtract(\tlendyear, \startyear))), 10))}
\draw (\yearcolumnwidth+2.5pt, \templength) -- (\yearcolumnwidth+7.5pt, \templength);
\node[year] (invisible-year-##1) at (\yearcolumnwidth, \templength) {};
}
\draw ($(invisible-year-##1.east)+(2.5pt, 0pt)$) -- ($(invisible-year-##1.east)+(7.5pt, 0pt)$) -- ($(entry-\entrycounter.west)-(5pt,0)$) -- (entry-\entrycounter.west);
}
\begin{tikzpicture}
\tikzstyle{entry} = [%
align=left,%
text width=\entrycolumnwidth,%
node distance=10mm,%
anchor=west]
\tikzstyle{year} = [anchor=east]
\tikzstyle{timelinerule} = [%
draw,%
decoration={markings, mark=at position 1 with {\arrow[scale=1.5]{latex'}}},%
postaction={decorate},%
shorten >=0.4pt]
\drawtimeline
}
{
\end{tikzpicture}
\let\startyear\@undefined
\let\tlendyear\@undefined
\let\yearcolumnwidth\@undefined
\let\rulecolumnwidth\@undefined
\let\entrycolumnwidth\@undefined
\let\timelineheight\@undefined
\let\entrycounter\@undefined
\let\ifnodedefined\@undefined
\let\ifnodeundefined\@undefined
\let\drawtimeline\@undefined
\let\entry\@undefined
}
\makeatother
\begin{document}
\begin{timeline}{1900}{1990}{2cm}{2.5cm}{5cm}{12cm}
\entry{1903}{Wilbur and Orville Wright fly the first powered airplane}
\entry{1914}{Assassination of Franz Ferdinand}
\plainentry{1917}{The October Revolution}
\entry{1928}{Discovery of Penicillin}
\plainentry{1929}{Stock Market Crash of 1929}
\entry{1941}{Attack on Pearl Harbor}
\plainentry{1944}{D-Day}
\entry{1945}{The Bombing of Hiroshima}
\plainentry{1947}{Creation of Israel as a Jewish State}
\entry{1963}{US president John F. Kennedy assassinated in Dallas}
\entry{1969}{The Moon Landing}
\plainentry{1989}{Fall of the Berlin Wall}
\end{timeline}
\bigskip
Text from: A Brief History of LaTeX http://www.xent.com/FoRK-archive/feb98/0307.html
\smallskip
\begin{timeline}{1974}{1985}{2cm}{7cm}{10cm}{0.45\textheight}
\entry{1974}{Donald Knuth stops submitting papers to the AMS because ``the finished
product was just too painful for me to look at''.}
\entry{1977}{Knuth begins his research on typography.}
\entry{1978}{Knuth delivers an AMS Gibbs Lecture entitled Mathematical Typography to the AMS membership at its annual meeting.}
\entry{1979}{Digital Equipment Corporation and the AMS jointly publish Knuth's TeX and METAFONT: New Directions in Typesetting.}
\entry{1980}{The first draft of Spivak's Joy of TeX is announced in TUGboat, vol. 1, no. 1.}
\entry{1982}{Spivak announces AMS-TeX at the joint math meetings.}
\entry{1982}{Version 0 of Spivak's Joy of TeX is released.}
\entry{1982}{Knuth releases dvitype, a model DVI driver.}
\entry{1983}{Lamport writes a LaTeX manual, the earliest known LaTeX manual in existence.}
\entry{1984}{Addison-Wesley publishes Knuth's The TeXbook, destined to become the definitive TeX reference.}
\entry{1984}{Lamport releases version 2.06a of the LaTeX macros.}
\entry{1985}{The Computer Modern (CM) fonts replace the American Modern (AM) fonts in TeX.}
\entry{1985}{Patashnik releases BibTeX version 0.98 for LaTeX 2.08. [``BibTeX 1.0'', TUGboat, vol. 15, no. 3, pp. 269--274, Sept. 1994.}
\end{timeline}
\end{document}
AT\&T, notAT and T.