inTime  = cumsum(rand(100,1));  %They show up randomly
outTime = inTime + rand(100,1) + 0.25;  % Stay for 0.25 to 1.25 hrs

inCount  = ones(size(inTime));  %Add one for each entry
outCount = ones(size(outTime))*-1; %Subtract one for each exit.

allTime  = [inTime; outTime]; %Stick them together.
allCount = [inCount; outCount];

[allTime, idx] = sort(allTime);%Sort the timestamps
allCount = allCount(idx); %Sort counts by the timestamps
allCount = cumsum(allCount); %total at any given time.

plot(allTime,allCount);%total at any given time.