Panning a coordinate system using the mouse (taking zooming in account)

Question

One of my projects contains a working custom coordinate system, which different objects are placed on. The coordinate system class stores the scale and the offset, which in turn affects the scale and location of all the objects in the coordinate system. While it works correctly, I am stumped on using the mouse to interact with the coordinate system. Specifically, dragging the mouse should pan the coordinate system, so that the coordinate system moves at the same rate as the mouse, which means that the amount the mouse changes the offset is multiplied by a factor relating to the zoom. What formula/algorithm would accomplish this?

I have provided two functions in my program: c(), which converts an object's coordinates into the canvas coordinate system coordinates (which takes into account scale and offset), and cRev(), which does the reverse. These appear to be in order:

    def c(self,pos): # canvas to screen coords.
    x = pos[0]+self.xOffset # change offsets
    y = pos[1]+self.yOffset

    x = (x - CENTER_X) * self.scale + CENTER_X # change scales
    y = (y - CENTER_Y) * self.scale + CENTER_Y

    return [int(x),int(y)]

def cRev(self,pos): # screen coords to canvas
    x = pos[0]
    y = pos[1]

    x = (x-CENTER_X) / float(self.scale) + CENTER_X
    y = (y-CENTER_Y) / float(self.scale) + CENTER_Y

    x -= self.xOffset
    y -= self.yOffset

    return [int(x),int(y)]

I have a version of the mouse algorithm that does not work as intended - the mouse does move at the same rate as the coordinate system; however, it "jumps" every time the mouse just starts to be dragged.

A shortened version is provided below: (Keep in mind that all the variables are global)

for event in pygame.event.get():
if event.type == pygame.MOUSEBUTTONDOWN:
    orig_x, orig_y = event.pos
    origXOffset = d.xOffset
    origYOffset = d.yOffset
    mouseDown = True

elif event.type == pygame.MOUSEMOTION:
    x = event.pos[0]
    y = event.pos[1]
    if mouseDown: # dragging
        xa,ya = d.cRev(event.pos)
        mousex,mousey = origXOffset + xa - orig_x,origYOffset + ya - orig_y
        d.xOffset += mousex
        d.yOffset += mousey
elif event.type == pygame.MOUSEBUTTONUP:
    mouseDown = False