get rid of the warning when analysing it

PNPieChart.m:137:33: warning: Passed-by-value struct argument contains
uninitialized data (e.g., via the field chain: 'origin.x')
Merge pull request #60 from congbo/master

get rid of the warning when analysing it
NSString -> NSNumber
Merge pull request #61 from cleexiang/master

NSString -> NSNumber