Band-pass Butterworth filter with Scipy.signal.butter

Question

I've tried the tutorial How to implement band-pass Butterworth filter with Scipy.signal.butter

I was just wondering if there was a way to improve the result at the beginning of the curve? Or calculate where it's "false".

Here is the area drawn on the figure I'm talking about, signal filtered with Butterworth:

I want to use this method to extract waves signal from current measurements.

Thanks.

Fanch