vertical colored lines on a slider matplotlib
The Slider resides in barpos, not in self.ax.
If you call self.ax.axvline(..), the axvline will be inside self.ax which is the subplots with the bars. If you want to have the axvlines in the Slider axes, you need to call barpos.axvline(). Better make barpos a class variable as well and then call
self.barpos.axvline()