I too have been struggling with charts since moving from OH1 to OH2 a few weeks ago. Some of my charts display, some so not. It seems that all of the ones that do not are monitoring ON/OFF events like motion sensors and such.
I am using rrd4j persistence, and the items I chart are stored everyMinute. This worked just fine in OH1.
Here is an example chart sitemap entry that does not work (ie displays a blank chart):
Chart item=basementLeakDetector period=D refresh=6000 service="rrd4j"
This is tracking a contact closure OPEN/CLOSE event.
Here is the item:
Contact basementLeakDetector "Leak Detector [MAP(en.map):%s]" <faucet> (gB, Sensors, Leak, gCharts) {insteonplm="36.B4.0C:F00.00.0A#contact"}
But I have other items that are Switches, and none of them display. I have to conclude that either the items are not stored in the rrd4j database, or they are not charted properly. It used to be that ON/OFF items were stored as 0 and 1 items in the database (as rrd4j can only store numbers). The log seems to indicate that this is the case:
2016-11-23 14:42:04.072 [DEBUG] [sistence.rrd4j.internal.RRD4jService] - Stored 'basementLeakDetector' with state 'OPEN' in rrd4j database (again)
2016-11-23 14:42:04.072 [DEBUG] [sistence.rrd4j.internal.RRD4jService] - Stored 'basementLeakDetector' with state '1' in rrd4j database
And the database file timestamps are updated - so it would seem to be stored. Also my switch states are persisted over restarts. So it must be a charting issue.
Checking the charting, if I enter:
http://ip:port/chart?items=basementLeakDetector&period=W
I get a blank chart, but if I enter:
http://ip:port/rrdchart.png?items=basementLeakDetector&period=W
Then I get a chart displayed as expected.

Anything that is a number will plot with the 'chart" http call, but it requires a call to rrdchart.png to chart switch items. “chart” will not plot them.
I have to conclude that the charting of ON/OFF items is broken in the latest build of OH2 (and has been for a while) - I upgraded to the nightly build last night. I can work around it, but it would be easier if it was fixed. I will enter this as a github issue.