Custom formatting for DateListPlot

Question

I'd like to control more aspects of a DateListPlot, for example: shading for weekend days, and/or indicators for daytime/nighttime areas.

By way of illustration, here's a simple example of a set of time data points (recent questions on mathematica.stackexchange):

questions = 
  First[Rest[
    Import["http://api.stackexchange.com/2.0/questions?page=1&\
pagesize=100&order=desc&sort=creation&site=mathematica", "JSON"]]];

questionTimes = 
  Cases[questions, HoldPattern["creation_date" -> value_] :>  value, 
   Infinity];

hours = (Mod[#, 86400] /86400) 24  & /@ questionTimes ;

DateListPlot[Transpose[{questionTimes, hours}], Filling -> Bottom, 
 GridLines -> False, Frame -> {True, True, False, False}]

and the plot is like this:

I can't see how to show the necessary information along the x-axis, nor how to shade different areas of the graph to show day/night.

Edit: I now realise that the date/times returned by the SE API are in Unix Epoch (1970), and I hadn't noticed because I wasn't able to see the day numbers or years on my first attempt at a plot...

Answer 1

Somehow DateListPlot is resistant to many styling options. Without fiddling with the internals, here is a starting point.

WeekendQ[date_] := 
 With[{d = DateString[date, "DayName"]}, 
  MatchQ[d, "Saturday" | "Sunday"]]
weekStyle = {Blue};
weekendStyle = {PointSize -> .015, Directive[Red]};
styleList = 
  Map[If[WeekendQ[#], weekendStyle, weekStyle] &, questionTimes];
DateListPlot[Partition[Transpose[{questionTimes, hours}], 1],
 PlotStyle -> styleList, Filling -> Bottom, GridLines -> False,
 FillingStyle -> ColorData[1][1],
 Frame -> {True, True, False, False},
 PlotRange -> {0, 24}, 
 Prolog -> {LightGray, Rectangle[Scaled[{0, 0}], Scaled[{1, 6/24}]], 
  Rectangle[Scaled[{0, 20/24}], Scaled[{1, 1}]]}]

Looks like:

Answer 2

Here is an alternative approach which uses colour coding of date labels to indicate Weekday/Weekend ( Green/Red ) and colour shade to indicate daytime/nightime ( Lighter/Darker ).

I've dropped some of the points and applied a minimum separation to help make the plot more legible in the limited display space here.

Functions

(* Check for the type of day of the week *)
WeekdayQ[dateString_String] := With[{weekend = {"Sat", "Sun"}}, 
  And @@ (StringFreeQ[dateString, #] & /@ weekend)]

(* Check for time of day, adjust for time of year and location *)
DaytimeQ[dateString_String] := With[{daylight = Range[8, 18]}, 
  MemberQ[daylight, (DateList@dateString)[[4]]]]

(* Apply colour adjustments to date strings to reflect weekday/time *)
ColorizeDate[dateString_String] := Style[dateString, 
  If[DaytimeQ@dateString, Lighter, Darker]@
   If[WeekdayQ@dateString, Darker@Green, Red]]

(* Ease points apart enough to make the text legible  *)
Options[Relax]={MinSeperation->0.03};
Relax[data_,OptionsPattern[]]:=With[{sep=OptionValue@MinSeperation(Max@data-Min@data)},
Flatten[{First@data,First@data +Accumulate[If[#<sep, sep,#]&@Differences@data]}]]

Prep data for plotting

(* points thinned to allow plot to be readable within the online space constraints *)
questionTimes = 
  Reverse@Cases[questions, 
     HoldPattern["creation_date" -> value_] :> value, 
     Infinity][[Select[Range@Length@questions[[2]], OddQ]]];

shift = DateDifference[{1900, 1, 1}, {1970, 1, 1}]*24*60*60;

hours = (Mod[#, 86400]/86400) 24 & /@ questionTimes;

questionData = {ToDate /@ (questionTimes+shift), hours}\[Transpose];

Plot the data

With[{labels = 
   Rotate[ColorizeDate@DateString@#, (3 \[Pi])/2] & /@ questionData[[All, 1]], 
  softCoords = Relax[AbsoluteTime /@ questionData[[All, 1]], MinSeperation -> 0.01]}, 
 ListPlot[{softCoords, questionData[[All, 2]]}\[Transpose], 
  Ticks -> {{softCoords, labels}\[Transpose], Automatic}, 
  Filling -> Axis, AxesOrigin -> {First@softCoords, 0}]]