Updated API.md for %q time-series tick-label option.

API.md

@@ -585,7 +585,7 @@ xaxis: {
 ```
 
 This will result in tick labels like "2000/12/24". A subset of the
-standard strftime specifiers are supported:
+standard strftime specifiers are supported (plus the nonstandard %q):
 
 ```js
 %a: weekday name (customizable)
@@ -596,6 +596,7 @@ standard strftime specifiers are supported:
 %I: hours, 12-hour time, zero-padded (01-12)
 %m: month, zero-padded (01-12)
 %M: minutes, zero-padded (00-59)
+%q: quarter (1-4)
 %S: seconds, zero-padded (00-59)
 %y: year (two digits)
 %Y: year (four digits)