Find inflection point on a step response graph

Hi, I'm brand new to matlab and will appreciate detailed response. I try to find infection point of a graph created from step input response in multisim. I have approx. 5000 points which create the graph. I need to find inflection point of a created curve to be able to add a tangent at infection point which finally will let me find the values where the tangent crosses x axis. I have attached multisim data and also the pdf showing roughly what I try to find out. Any help much appreciated.

[blpanther]

this is my excel data