Question

how to find cumulative frequincy

Answer 1

Suppose your sample space has four ordered elements, A, B, C, D, each with a frequency Fa, Fb, Fc, Fd respectively.

If you were plotting frequency, you'd have four equi-spaced point on the x-axis for A,B,C and D. And then for each you'd plot Fa, Fb, Fc and Fd respectively.

For cumulative frequency you plot instead the sum of the frequency up to that point:
Fa
Fa + Fb
Fa + Fb + Fc
Fa + Fb + Fc + Fd

More generally, if x_j, j = 1,2, ..., n are the ordered members of the sample space with respective frequencies f_j, then the cumulative frequency C_j is given by
\[ C_j = \sum_{i=1}^j f_j \]