Is there any way to achieve override the function that creates or render the labels?
No. The Qt Charts library is rigid and completely non-customizable. You get what you get, and if you don't like it, too bad. There are no virtual functions you can override, no way to add a new series type, no way to add a new chart type, no way to do anything that doesn't come out of the box.

You should take a look at QCustomPlot or Qwt if you want a plotting library with more flexibility.