plotting histogram together with variance
The problem is that you are plotting two things of different scale on the same axis.
Your variance appears to be 6*10^9 while your counts are in the hundreds. Therefore the bins are so small you can no longer see them. 

You should probably use an vline instead of an hline to get the line on the x-axis

plt.axvline(var)


Here is a sample:

import seaborn as sns
import matplotlib.pyplot as plt
diamonds = sns.load_dataset('diamonds')
plt.hist(diamonds['carat'])
plt.axvline(np.var(diamonds['carat']), color='red')
plt.show()






If you really want to do this on the y axis you should use a second y axis. 

Change this

plt.axhline(var)


to this

ax2 = plt.gca().twinx()
ax2.axhline(var)


Check the documentation for more info

Otherwise consider using multiple plots since it might be really confusing to have a line with a different scale on a histogram.