Update ViewPortHandler.swift (#3143)
Update ViewPortHandler.swift (#3143)

fix a small bug
liuxuan30/ios-charts,MAARK/Charts,MAARK/Charts,MAARK/Charts,FreddyZeng/Charts,icecrystal23/ios-charts,liuxuan30/ios-charts,jum/Charts,danielgindi/ios-charts,icecrystal23/ios-charts,danielgindi/ios-charts,FreddyZeng/Charts,jjatie/Charts,danielgindi/Charts,jjatie/Charts,jum/Charts,MAARK/Charts,danielgindi/ios-charts,danielgindi/Charts,FreddyZeng/Charts,icecrystal23/ios-charts,liuxuan30/ios-charts,icecrystal23/ios-charts,jjatie/Charts,danielgindi/ios-charts,danielgindi/Charts,liuxuan30/Charts,jjatie/Charts,liuxuan30/Charts,jum/Charts,FreddyZeng/Charts,liuxuan30/Charts,jum/Charts,danielgindi/Charts,liuxuan30/Charts,liuxuan30/ios-charts