don't render small graphs that are not visible
don't render small graphs that are not visible

This makes stats page tolerable on slower browsers/machines.

Change-Id: Ie31e2c1271e678bb2d3f60dc20fb7135177ab3d1
Reviewed-on: http://review.membase.org/4445
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Reviewed-by: Dale Harvey <491ccac09a36ad1e38b698f273b84fd8efc31071@arandomurl.com>
Reviewed-by: Benjamin Young <399aded95f850edb83f13cae14dbe3e10ed19071@bigbluehat.com>

vzasade/ns_server,mschoch/ns_server,ceejatec/ns_server,membase/ns_server,t3rm1n4l/ns_server,vzasade/ns_server,t3rm1n4l/ns_server,nimishzynga/ns_server,membase/ns_server,mschoch/ns_server,pavel-blagodov/ns_server,ceejatec/ns_server,membase/ns_server,daverigby/ns_server,daverigby/ns_server,vzasade/ns_server,mschoch/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,daverigby/ns_server,ceejatec/ns_server,mschoch/ns_server,membase/ns_server,membase/ns_server,ceejatec/ns_server,t3rm1n4l/ns_server,pavel-blagodov/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,mschoch/ns_server,pavel-blagodov/ns_server,daverigby/ns_server,mschoch/ns_server,vzasade/ns_server,ceejatec/ns_server,ceejatec/ns_server,membase/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,daverigby/ns_server,pavel-blagodov/ns_server