Marking a specific point on a PDF graph

How can I mark a point with an (x,y) coordinate on this plot. I have attached my code. A user enters an academic class 2A 2B 3A 3B,, etc and then a gpa. I want it to find the entered gpa and mark a point on it on the graph.

Here is my code:

%This program tells the probability of getting a GPA given all of the
%department's students and their gpa's
    clear;
    close all;
    clc;
% Asks what your academic class and gpa are and inputs into matrix
    prompt = {'Academic Class','GPA'};
    dlgtitle = 'GPA Probability Calculator';
    dims = [1 35];
    definput = {'Exp. 1B','Exp. 4.0'};
    answer = inputdlg(prompt,dlgtitle,dims,definput)
%This imports the GPA & Academic class data from fall 2016 and puts it in
%an array
    grades = readtable('2016grades.xlsx');
%Read table and find corresponding GPAs to academic class
    celldisp(answer)
    mask = strcmp(grades{:,1}, answer{1});
    retrieved_class = grades{mask, 2};

%Plot a PDF
[p,x] = hist(retrieved_class); plot(x,p/sum(p)); %PDF

Here's is my output if I enter 3B for my academic class.

Answer 1

[Ridwan Alam]

gradeIn = str2num(answer{2});

[p,x] = hist(retrieved_class);
p = p/sum(p);
figure;plot(x,p)
hold on; plot(x(x==gradeIn),p(x==gradeIn),'ro')