or which y lies in the range N1 /D to N1, where D is an integer and the base of the logarithms for the logarithmic independent axis,said apparatus comprising signal processing means arranged to form for each value of x two values y1 and y2 where y1 =INT(yf1 +1/2) y2 =INT(yf2 +1/2) yf1 =N1 /D???D.sup.(x-1/2)/N.sbsp.2 yf2 =yf1 ???D1/N.sbsp.2 and where INT represents the integral part of, means arr