# Create a dataframe
import pandas as pd
df = pd.DataFrame({'group':list(map(chr, range(65, 85))), 'values':np.random.uniform(size=20) })

# Reorder it following the values:
ordered_df = df.sort_values(by='values')
my_range=range(1,len(df.index)+1)

# Make the plot
plt.stem(ordered_df['values'])
plt.xticks( my_range, ordered_df['group'])