fix #2099, avoid crash when some chart only allow 1 data set but people will set more than that
fix #2099, avoid crash when some chart only allow 1 data set but people will set more than that

danielgindi/Charts,SereivoanYong/Charts,ConcepterDev/ios-charts,jjatie/Charts,icecrystal23/ios-charts,hanquochan/Charts,ConcepterDev/ios-charts,jum/Charts,MAARK/Charts,ConcepterDev/ios-charts,icecrystal23/ios-charts,liuxuan30/Charts,FreddyZeng/Charts,danielgindi/ios-charts,SereivoanYong/Charts,hanquochan/Charts,jum/Charts,liuxuan30/Charts,danielgindi/ios-charts,danielgindi/Charts,MAARK/Charts,MAARK/Charts,jum/Charts,SereivoanYong/Charts,liuxuan30/ios-charts,jjatie/Charts,icecrystal23/ios-charts,liuxuan30/ios-charts,liuxuan30/ios-charts,jjatie/Charts,jum/Charts,danielgindi/ios-charts,danielgindi/Charts,liuxuan30/Charts,icecrystal23/ios-charts,FreddyZeng/Charts,jjatie/Charts,hanquochan/Charts,danielgindi/Charts,MAARK/Charts,danielgindi/ios-charts,FreddyZeng/Charts,liuxuan30/ios-charts,liuxuan30/Charts,FreddyZeng/Charts,hanquochan/Charts