Xcode编译警告Assigning to 'id _Nullable __strong' from incompatible type 'ChartDe

ios-charts的barChart更改柱子上的文字格式
NSNumberFormatter *formatter = [[NSNumberFormatter alloc] init];
formatter.maximumFractionDigits = 0;
barChartData.valueFormatter = [[ChartDefaultValueFormatter alloc] initWithFormatter:formatter];

不使用
barChartData.valueFormatter = [[ChartDefaultAxisValueFormatter alloc] initWithFormatter:formatter];

Xcode回报出警告
Assigning to ‘id _Nullable __strong’ from incompatible type ‘ChartDefaultAxisValueFormatter *’