Get unexpected value form stringForValue

It work fine when linechartview first time appear. The issue occur when I change dataset, dataEntry and valueFormatter then update chart, the function "stringForValue" return old value and mess the chart.

before update chart, stringForValue give values like
value=4732.000000
value=4733.000000
value=4735.000000
value=4736.000000
.
.

after update chart, stringForValue give values like
value=4732.000000 <---- unexpected value
value=4733.000000
value=789.000000 <---- correct value
value=789.200000
.
.

Is something I did not call or use?