MATLAB: Problem while plotting twice in a figure

axisplotplotting

I have 2 imported columns from excel (time and value), that contain more than 10000 rows, I plotted them and also the average of the whole values.
Now I want to plot both the time on the x axis and the values on the y axis with the average line (see second code) in one figure, I get the following error:
Values plotted against x-axis must be categorical values. To create categorical values, use the categorical function.
when I convert the value to a categorical one, it says: " Invalid data type. First argument must be numeric or logical."
Can Anyone tell me where the problem can be?
%the first correct code:
nexttile
plot(time,value)
nexttile
plot([0 size(value,1)],[avg avg], 'y-', 'LineWidth',2);
%The second one where I get the error:
nexttile
hold on
plot(time,value)
plot( [0 size(value,1)],[avg avg], 'y-', 'LineWidth',2);
hold off

Best Answer

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
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