Plotting histogram with text using python

I am trying to plot a histogram in python, and add text on the right upper corner.

here I am creating and plotting the histogram:

sample = stats.poisson.rvs(loc = 0,mu = lamda, size = 10001)
plt.hist(sample)
pd.DataFrame(sample).hist(bins=58, 
                          figsize=(9,9),
                          edgecolor="k", linewidth=1)

Now, I am trying to plot the mean and median in the right upper corner:

plt.text(0.8, 0.9, s = 'mean = {0}'.format(round(np.mean(sample), 2)))
plt.text(0.8, 0.8, s = 'median = {0}'.format(np.median(sample)))

and here is the screenshot of the output:

As you can see, the x and y values of the text are coordinate values.

How can I pass relative x and y values (to place the text in the upper right corner)?

You need to specify which coordinate system you want to use, otherwise it will automatically use the data coordinate system. In your case you want to use ax.transax.

plt.text(0.8, 0.9, s = 'mean = {0}'.format(round(np.mean(sample), 2)),transform=ax.transAxes)
plt.text(0.8, 0.8, s = 'median = {0}'.format(np.median(sample)),transform=ax.transAxes)

I suggest you to read this

You can also find an example in the matplotlib text documentation