Try this out: Since I didn't have your variable class, I had to assume you're dealing with time-stamps here. This works for those:
t1 = datetime(2020,8,12,13,11,24); %start time
t2 = datetime(2020,8,12,13,18,36); %end time
time5 = t1:seconds:t2; %create a vector of time points from the start time to the end time
v5 = randn(size(time5)); %create random y-axis data
plot(time5,v5) %plot data
hold on; %hold the current figure so the first plot doesn't erase when you do the 2nd plot
avg5 = mean(v5); %the average value of v5
x2= [time5(1) time5(end)]; %x-values for the average line
plot(x2,[avg5 avg5], 'y-', 'LineWidth',2); %plot the avg line
title('value 5 by time plot') %title the plot
legend('Raw data','Average line') %add a legend to differentiate the two data sources
ax = gca; %get current axis
x_step_size=100; %step size between labels in seconds
ax.XAxis.TickValues = t1:seconds(x_step_size):t2; %adjust the tick values to the correct spacing
