y holds proportions between 0 and 1. Plot it against x and format the y-axis tick labels as percentages, so 0.25 reads as 25%.

Return the Axes. Multiplying the data by 100 is the wrong fix: the values are proportions, and it is the labels that should change.

matplotlib.ticker.PercentFormatter is importable inside your solution; pass xmax=1 so it knows 1.0 means 100%.