Straight line in learn.sched.plot()


I was trying out resnet34 architecture on Happy House dataset on Kaggle and I found that I am getting a straight line with slope -1 using learn.sched.plot(). I used a rate just smaller than the highest value on the plot (0.3) and it worked well (I got 97% accuracy) but I wanted to know why I got that kind of plot and if I am making any mistake.
Here is the link to the kaggle kernel for reference.

Thank you.