is a timeline and graph-layout in one
timegraph is in of a basket of visualizations, collectively dubbed examine which can be invoked by selecting their view in a query string.
a more primitive scale a simple one-dimensional display
with overlapping values you might want a histogram which instead puts things into bins
any value which can be cast to float works with these views, including Time values, which are transparently converted to seconds-since-epoch
to instantiate a timegraph, add view=tg to query string:

you may notice there are a lot of lines (arcs) in this. indeed - emails often include references to all parent-messages back to thread-origination, but we're mainly interested in the previous message. timegraph accepts an arc attribute to select a property for arc display.
first i'll pull up the last 555 emails in the "top threads" view to find a thread
and look at direct replies

node labels are another option, a comma-seperate list of RDF predicate-names, common prefixes shrunken is fine sioc:name,dc:date
groups are probably self-explanatory, in this next view group-by is a channel property of a message, and non-time axis is set to a lineCount property generated by log scanning code

in some cases, resources contain links to next/previous but not always. a named-filter was created to decorate the runtime model with these links based on time fields, and the chart begins looking like a more familiar line-graph of time-series data
there are a few more features crammed in this view, the time-axis is also overridable to be anything else, in which case you have a graph-layout view with 2-dimensions beyond arcs for resource placement, all configurable from query-string contortions
view was implemented using a standard style which allows a faceted-filterer to turn on or off parts of the graph based on external filter parameters
tafting visualization is another use case