I have some data in my app using Qwt Plot and i want to measure some delta values using my mouse in my plot.

eg:
given:
point 1 (1,3)
point 2 (2,4)
point 3 (3,1)

now if i point my mouse to point 1 (click) then point 2 (click)

then i should show:

delta x = 1 and delta y = 1
how can i implement this in Qwt? please enlightened me

baray98