Suppose you want to plot the function f - as in
Then plotting it "upside-down" can be achieved by using the value=label construct of the tickmarks plot option, together with multiplying the function by (-1), as in
tVals:=[-100, -66, -33, 0]:
tLis:= [seq(j=convert(abs(j), string), j in tVals)]:
plot(-f, 0..10, axis=[tickmarks=tLis]);
The drawback of this approach is that you have to "know/guess" the values of the tickmarks.
I am reasonably sure that one could automate this if necessary: eg if the x-range of the plot is known, then max and min values of the function can be calculated. Then a suitable range for y-axis tickmarks, can be computed which can then be translated to the appropriate labels