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The scale of the x-axis (Format: HH:MM:SS)

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Ramo Rafsel
Ramo Rafsel on 10 Aug 2020
Edited: Ramo Rafsel on 18 Aug 2020
I have 2 imported columns from excel that I plotted already, the time column ( the format HH:MM:SS) is on the x-axis and the value column is on the y-axis.
The problem that I am trying to solve since both the columns are 1645 long (or better said: they contain 1645 rows), so I want to change the x axis scale on the graph to make the distance between the times somehow longer, otherwise it looks like a long unwanted line under the x-axis.
Any Ideas how to change the x-axis scale on the graph for a better appearance?
The second hurdle that I am facing, I keep getting the error below whenever I try to plot the average line of the value column on the same graph where the time and the value column are plotted. Maybe because the x and y axis are already used?
avgline= sum(value_column)/size(value_column),1); %the value_column and time column are already defined from the workspace)
hold on
title('Value 6')
hold off
%The Error that I get:
%Values plotted against x-axis must be categorical values. To create categorical values, use the categorical function.


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Accepted Answer

Shae Morgan
Shae Morgan on 10 Aug 2020
Edited: Shae Morgan on 10 Aug 2020
try looking into the 'xtick' property to specify which x-ticks you want to show. it'll suppress the rest of them
x=1:10; %demo time values
y=1:10; %demo data
plot(x,y) %create figure
set(gca,'xtick',[1 3 6 10]) %manipulate x-axis to only show some values


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Ramo Rafsel
Ramo Rafsel on 14 Aug 2020
@Shae Morgan Thanks a lot for your help. you gave me a solution for the time stamp problem. the x values are the time in HH:MM:SS format, and it is categorical. If I correctly understood your question about the variable type of the x values.
One thing that I couldnt understand by comparing the picture that I added and your plot, my y-axis has another scale. While yours is between 4 and -4.
I tried using the value column v5 instead of time5 because I wanted to plot the value column on the y axis. But I get a figure with two lines.
And regarding this line and also t1 and t2 variables (I added the two variables with the first and last hour (in HH:MM:SS), as you did), do I have to create random y-axis data, when I already have the value column that contains the data for the y-axis?
v5 = randn(size(v5)); %create random y-axis data
Shae Morgan
Shae Morgan on 14 Aug 2020
you don't have to make up random data to use for your y-axis. Just use the variable you have. You didn't provide any of your data, so I had to make-up numbers for the y and x axes to demonstrate the fix for the x-axis. You substitute those lines of code for what you already have and it should work.
%You don't need this code if you already have a time5 and v5 variable
%I added this code in order to demonstrate the fix
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
% --------------^^^^^----- don't use this code if you already have your time5 and v5 variables!
%end set-up section
%use this code with your original time5 and v5 data that you already have stored as variables
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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