optimizing of new statistics render code

-fix: legend disappeared on swipe/ tabchange on Android 2.1 devices.
-increased text size slightly
-turned numbers on y-axis to 90° and reduced necessary space on the left and right side
-optimized CreateChartTask to only run one task at a time and let tasks be cancelable (before they have been in doInBackground()) to reduce strain when changing statistics time frame when tasks are still running.