TankBar: use the itemPos as intended

Which actually makes the code much clearer as now the object is at the
correct spot on the canvas and the positions inside are relative to that.
No more magic gradiants starting at "92"

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
TankBar: hook into prefs and make ppGraphs resize accordingly

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>