Purpose:
   Provide a persistent URL to graphs of the most recent USGS real-time
     water data suitable for inclusion in a HTML image tag on user-defined
     web pages.

Usage:
     
  https://waterdata.usgs.gov/nwisweb/graph?<args>

   ...where <args> are:

       agency_cd=   Agency code for site
                       (if omitted default = USGS)

       site_no=     USGS site number to match
                       (required argument -- error if omitted)

       parm_cd=     USGS parameter code
                       (required argument -- error if omitted)

       period=      Include data for the previous "period" days
                       (if omitted default = 7 unless begin/end_date supplied)

       begin_date=  Begin date for graph period
                       (will not occur with period in site-page link)

       end_date=    End date for graph period
                       (will not occur with period in site-page link)
                       
       help         Display this text



 Example URLs: 

   https://waterdata.usgs.gov/nwisweb/graph?agency_cd=USGS&site_no=06025500&parm_cd=00060&period=7
   https://waterdata.usgs.gov/nwisweb/graph?agency_cd=USGS&site_no=06025500&parm_cd=00060&begin_date=2012-04-20&end_date=2012-04-27


 Example image tag:

   <img src="https://waterdata.usgs.gov/nwisweb/graph?site_no=06025500&parm_cd=00060" width="576" height="400" alt="USGS Water-data graph" />