Python & Matplotlib: how to speficy unit when ploting histogram?

Question

I have a datafram like this:

I want to plot the df.speed, its unit is 1. If I do df['speed'].hist(), the grouping would make them too broad, like this:

Here the grouping for each rectangle is 10, How can I make it into 1?

I only see there is a bin parameter, but it is the overall count of the groups.