KReporter - How to make the chart display field as date

I need to graph the amount of leads per day in KReport. How to configure the graph to display date and not datetime. In the present tab in the “override type” option when I change to “date”, the report below the chart is displayed correctly, but the chart keeps the display as datetime.

I found a documentation of the sugar in which they use the formula date (subStr (toString ($ date_modified), 0,10)), however the field is displayed “NaN / NaN / NaN”
Has anyone ever had this problem? Any suggestion?