You can do it in two ways:
a) Slice the DataFrame so you have a new DataFrame and can plot that one:
new_df = old_df[old_df[‘HelfulnessNum’ ≥7]]
plt.hist(new_df) etc. etc
b) Customize the range whilst creating the histogram:
plt.hist(old_df, range=[7, max])
I don’t know what your max value is so I just said max above. See: http://stackoverflow.com/questions/12125880/changing-default-x-range-in-histogram-matplotlib
Hope this helps!