definepapersize[BookPaperSize][
  width=8in,
  height=6in,
]

setuppapersize[BookPaperSize]

% Specify the width and height here because the inline figures need to
% maintain aspect ratio while setting a pleasing width.
startsetups[BookIllustrationSetups]
  setlayerframed[BookIllustrationLayer][
    frame=off,
    x=zeropoint,
    y=1.25in,
    width=makeupwidth,
    background=BookTimelineOverlay,
  ]{}
stopsetups

setupbackgrounds[page][
  background=BookIllustrationLayer,
  setups=BookIllustrationSetups
]

definelayer[BookIllustrationLayer][
  width=paperwidth,
  height=paperheight,
]

defineoverlay[BookTimelineOverlay][useMPgraphic{BookTimelineGraphic}]

startuseMPgraphic{BookTimelineGraphic}
  % Define constants
  numeric EVENT_BEGAN, TIME_OFFSET;
  EVENT_BEGAN := 13799;
  TIME_OFFSET := 1.25in;
  PATH_THICKNESS := 2pt;

  % The book timeline title macro contains a number representing when the
  % event transpired and additional information.
  string sectionTitle, sectionTitleDigits;
  sectionTitle := "getspecificstructuretitle{3}";
  sectionTitleDigits := "";

  % Extract only digits and decimals from the book timeline title macro.
  for i = 0 upto length( sectionTitle ):
    string sectionChar;
    sectionChar := substring( i, i + 1 ) of sectionTitle;

    % A space indicates that the number has no more digits.
    if sectionChar = " ":
      break;
    fi;

    % Concatentate all the digits together.
    if ((sectionChar >= "0") and (sectionChar <= "9")) or (sectionChar = "."):
      sectionTitleDigits := sectionTitleDigits & sectionChar;
    fi;
  endfor;

  % Convert the digits from a string to a numeric value.
  numeric eventTime;
  eventTime := 0;

  for i = scantokens( sectionTitleDigits ):
    eventTime := i;
  endfor;

  numeric eventX;
  eventX := 1 - (eventTime / EVENT_BEGAN);

  % Draw the line.
  draw (TIME_OFFSET, .5*overlayheight) -- (overlaywidth, .5*overlayheight)
    withpen pencircle scaled PATH_THICKNESS;

  % Draw starting |.
  draw (TIME_OFFSET, 0) -- (TIME_OFFSET, overlayheight)
    withpen pencircle scaled PATH_THICKNESS;

  % Draw ending |.
  draw (overlaywidth, 0) -- (overlaywidth, overlayheight)
    withpen pencircle scaled PATH_THICKNESS;

  % Draw the timeline's dot relative to the event on the timeline.
  filldraw fullcircle scaled 9
    shifted(
      TIME_OFFSET + eventX * (overlaywidth - TIME_OFFSET), .5*overlayheight
    );
stopuseMPgraphic

starttext
  chapter[title={Inflation Theory},reference={inflation-theory}]
  section[title={13,799 ± 0.021},reference={section}]
  input ward

  chapter[title={First Stars},reference={first-stars}]
  section[title={13,689 ± 70},reference={section-1}]
  input ward
stoptext