Your question has nothing to do with an SQL query, it is simply a means to end. What you are really asking is how to change the text labels on a bar chart in pylab. The docs for the bar chart are useful for customizing, but to simply change the labels here is a minimal working example (MWE):
import pylab as plt
DayOfWeekOfCall = [1,2,3]
DispatchesOnThisWeekday = [77, 32, 42]
LABELS = ["Monday", "Tuesday", "Wednesday"]
plt.bar(DayOfWeekOfCall, DispatchesOnThisWeekday, align='center')
plt.xticks(DayOfWeekOfCall, LABELS)
plt.show()
