Document --period associated behaviour.

README

@@ -198,6 +198,52 @@ Portability note: the use of paths containing directory elements inside
 configuration files is discouraged, as it's not portable. This may change
 later (ex: deciding that import paths are URLs and applying their rules).
 
+Periods
+=======
+
+When providing the `--period` argument, two related settings are affected:
+
+- the period represented by each point in a graph (most important for the
+  hit graph, as it represents the number of hits per such period)
+
+- the period represented by each column in per-period tables (status codes
+  per date, hits per day...)
+
+Also, when `--period` is not provided, apachedex uses a threshold to tell
+when to switch to the larger period. That period was chosen to correspond
+to 200 graph points, which represents a varying number of table columns.
+
+.. table :: Details of `--period` argument
+
+  ======== ======= ======= ============== =========================
+  --period graph   table   to next period columns until next period
+  ======== ======= ======= ============== =========================
+  day      hour    day     200 hours      9 (8.3 days)
+  week     6 hours week    1200 hours     8 (7.1 weeks)
+  month    day     month   5000 hours     7 (~6.7 months)
+  quarter  7 days  quarter 1400 days      16 (15.3 weeks)
+  year     month   year    (n/a)          (infinity)
+  ======== ======= ======= ============== =========================
+
+"7 days" period used in `--period quarter` are not weeks strictly
+speaking: a week starts a monday/sunday, pendending on the locale.
+"7 days" start on the first day of the year, for simplicity - and
+performance. "week" used for `--period week` are really weeks, although
+starting on monday independently from locale.
+
+When there are no hits for more than a graph period, placeholders are
+generated at 0 hit value (which is the reality) and 100% apdex (this is
+arbitrary). Those placeholders only affect graphs, and do not affect
+averages nor table content.
+
+Because not all graph periods are actually equal in length (because of
+leap seconds, DST, leap years, year containing a non-integer number of
+weeks), some hit graph points are artificially corrected against these
+effects. Here also, the correction only affects graphs, neither averages
+nor table content. For example, on non-leap years, the last year's
+"7 days" period lasts a single day. Ploted hit count is then multiplied
+by 7 (and 3.5 on leap years).
+
 Performance
 ===========
 
@@ -232,11 +278,6 @@ For better performance...
 Notes
 =====
 
-When there are no hits for more than a graph period, placeholders are
-generated for 0 hit (which is the reality) and 100% apdex (this is
-arbitrary). Those placeholders only affect graphs, and do not affect
-averages nor table content.
-
 Loading saved states generated with different sets of parameters is not
 prevented, but can produce nonsense/unreadable results. Or it can save the day
 if you do want to mix different parameters (ex: you have some logs generated